The Vascular System of Octoehaetus thomasi

By

Maurice Bleakly, M.Sc.

University of Otago, New Zealand.

With Plates 8 and 9.

CONTENTS.

PAGEI N T R O D U C T I O N 2 5 1

P R E V I O U S W O R K 2 5 2

M E T H O D S . . . . . . . . . . . 2 5 4D E S C R I P T I O N O P T H E B L O O D - V E S S E L S 2 5 5

C O U R S E T A K E N B Y T H E B L O O D . . . . . . . 2 6 3

C O M P A R I S O N W I T H O T H E R T Y P E S . . . . . . 2 6 6

S U M M A R Y . . . . . . . . . . . 2 6 7

B I B L I O G R A P H Y . . . . . . . . . . 2 6 8

INTRODUCTION.

THE study of the vascular system in earthworms has attractedmany authors, but no detailed account has been published ofthe system in New Zealand earthworms or indeed in any ofthe Acanthodrilinae. The paper which follows deals with therelations of the blood-vessels and the circulation of the blood inO c t o e h a e t u s t h o m a s i Beddard, an earthworm belongingto this family. No account is given of the ultimate distributionof the vessels within the body-wall and in the nephridia since thisis essentially similar in all earthworms, and has been the subjectof careful description by Benham (5), Harrington (9), andBahl (1 and 2).

The work was carried out in the Zoology Department of theUniversity of Otago under the direction of Professor Wm. B.Benham, F.R.S., to whom I take this opportunity of expressingmy thanks for his helpful criticism and advice both in the pro-gress of the work and in the preparation of this manuscript.

252 MAURICE BLEAKLY

PREVIOUS WORK.

It appears that the first zoologist to give a fairly correctaccount of the course taken by the blood in an earthworm wasAntoine Duges,1 who in 1828 (p. 298) observed that the 'pulse'of the dorsal vessel sent the blood from behind forward, whereit enters the 'plexiform' vessels (i.e. lateral hearts) so as toreach the ventral vessels, of which he recognized both the ' sub-intestinal' and the 'subneural'. Further, he was led to believethat the blood passed upwards through the commissural vessels(his 'abdomino-dorsal branches') and so entered the dorsalvessel. These facts he established by cutting the worm into twoand noting the flow of blood from the cut vessels in each half.The earthworms studied were, of course, L u m b r i c u sand allied genera.

Beddard (3 and 4) gives a brief account of the major blood-vessels of 0 . m u l t i p o r u s and 0 . t h o m a s i in his descrip-tions of the species. He describes the dorsal vessel as completelydouble behind the gizzard. On this evidence, Stephenson (11)describes the dorsal vessel of earthworms as varying fromsingle, incompletely double—where the two limbs unite at thesepta—to a completely double condition, 0 . m u l t i p o r u sbeing taken as an example of this last type.

Benham (6) describes the major blood-vessels of the cepha-lized region of M a o r i d r i l u s u l i g i n o s u s , noting the doublenature of the dorsal and supra-intestinal vessels, and the seg-mental position of the heart and lateral vessel.

Bourne (7 and 8) described the vascular system and the cir-culation in Megasco lex c a e r u l e u s . Of anatomical interestwas his description of the hearts as both latero-intestinal, arisingdorsally from both dorsal and supra-intestinal vessels, andlateral, arising from dorsal vessel alone. Bourne studied thecourse of the blood by observing the contractions of the vessels,the arrangement and disposition of the valves, the conditionproduced by pinching the vessel between fine forceps, and thebleeding of severed vessels. He agreed with previous authors

1 Duges (1S28), "Recherches sur la Circulation, la Respiration et laReproduction des Annelides Abranches", 'Annales des Sciences naturelles',vol. xv.

VASCULAR SYSTEM OF OCTOCHAETUS 253

that the general course was forward in the dorsal vessel, down-ward in the hearts, and backward in the ventral vessel behindthe hearts, but considered that the flow was forward in theventral vessel anterior to the hearts. The lateral vessels areregarded as specializations of an intestino-tegumentary systempresent in each segment in the intestinal region—the flow inthese lateral vessels was in a forward direction.

Bourne's scheme of circulation allows a complete intestinalcirculation in each segment, blood leaving the dorsal vessel bydorso-tegumentary vessels to the integument, and passing thenceto the gut by intestino-tegumentaries and back to the dorsalvessel by the dorso-intestinals. The ventral vessel suppliedblood to the integument and to the intestine by ventro-tegu-mentary and ventro-intestinal vessels, but received no bloodposterior to the heart.

In D r a w i d i a (Moni l igas ter ) g r a n d i s , Bourne (8)described latero-intestinal hearts arising from the dorsal vesseland from transverse vessels communicating with the lateralvessels, there being no supra-intestinal in this species. Thecirculation is essentially similar to that of Megasco lexcaeruleus (7) save that there are no intestino-tegumentaryvessels and therefore no partially complete segmental circulation.

Harrington (9) studied the vascular system and circulationin L u m b r i c u s . His scheme of circulation is based entirelyon the direct observation of the pulsations seen in the vesselsof small specimens under a dissecting lens. Harrington's con-ception of the circulation, which differs widely from that ofBourne and which allows a complete segmental circulation, hasbeen superseded by the work of Johnstone and Johnstone.

Johnstone and Johnstone (10) give a careful account of thecirculation of L u m b r i c u s . Their methods are essentiallysimilar to those used by Bourne. They believe that Harrington'serrors can be attributed to his method of observation, sincelocalized contractions would produce momentary reversals offlow in the blood-vessels and might be mistaken for pulsations.Their scheme agrees with Bourne's in most essentials with theexception of the course in the dorso-tegumentary vessels. Thesevessels, they state, return blood to the dorsal vessel from the

254 MAURICE BLEAKLY

integument. Segmental circulation is impossible since the dorsalvessel collects blood from the organs, carries it forward to thecephalized region, where it is transferred by the hearts to theventral vessel, and thence back to the body and gut.

Combault (reported by Stephenson (11)) considers that the flowis backward throughout the ventral vessel. He constricted theventral vessel anterior to the hearts and found that posteriorlyto the constriction it filled as the hearts contracted and emptiedbetween contractions. He believed that this momentary reversalcould be explained as a mechanism for regulating pressure inthe ventral vessel and apparently ensuring an even flow ofblood.

Bahl (2) gives a careful description of the blood-vessels andthe course of the circulation in P h e r e t i m a . The generalscheme of circulation is similar to that in L u m b r i c u s as de-scribed by Johnstone and Johnstone. Posteriorly to the hearts noblood leaves the dorsal vessel, anteriorly to the heart all vesselsassociated with the dorsal vessel carry blood away from it.Ventro-tegumentaries and ventro-intestinals arise from theventral vessel in every segment, and carry blood to the integu-ment and the gut. The flow in the anterior portion of the ventralvessel is forward. Blood is collected from the anterior region byvessels communicating with the lateral vessel in which the flowis backward.

METHODS EMPLOYED FOB THE STUDY OF THE CIRCULATION

The largest specimens obtainable of 0 . t h o m a s i werechosen. After narcotization, and in many cases a subsequenthypodermic injection of strophanthin, these were dissected fromthe dorsal, lateral, or ventral surface under physiological saltsolution, and studied by means of a low-power binocular dis-secting microscope. Strophanthin has the effect of increasingthe blood-pressure and making the course of circulation easierto follow, and of causing the vessels to be dilated and so renderingsmall vessels easier to detect.

The course taken by the blood in the various vessels wasdetermined by the following methods:

(1) The direction of waves of contraction was noted. This

VASCULAR SYSTEM OF OCTOCHAETUS 255

method was found to be applicable only to the pulsatile vessels,i.e. the dorsal vessels and hearts.

(2) The action of the valves was studied where they werepresent, and the course that the blood must take to pass themascertained.

(8) The vessels were closed by means of clips made from finespring wire. Increase of pressure on one side of the clip wasshown by dilatation of the vessel, and decrease of pressure bypartial draining of the blood.

(4) The vessels were cut or broken. More profuse bleedingfrom one end than from the other gave evidence of greaterpressure on that side of the cut.

(5) An attempt was made to introduce traces of pigment tothe vessels and to watch its subsequent distribution. A syringefitted with a fine glass capillary-cannula was used; but, asthe force required to overcome the capillarity becomes verygreat as the diameter of the tube is decreased, it was not foundpossible to use tubes fine enough to enter any but the dorsalvessel. As pigments indian ink and methyl violet were employed,the former proving less satisfactory as it tended to deposit solidparticles and block the vessel.

DESCBIPTION OF THE BLOOD-VESSELS.

As in all earthworms there is a considerable difference betweenthe blood-vessels in the cephalized and intestinal regions. Inthe intestinal region the system is relatively simple and, as itis repeated metamerically, may be considered to represent thetypical arrangement of the blood-vessels. It is convenient todescribe the vessels in this region first, as both Harrington andBahl have done, and then to pass on to the more modifiedanterior region.

Segments Posterior to Segment 20.

Peri- intest inal Plexus.This plexus consists of a network of blood-channels lying

between the epithelial and muscular layers of the gut-wall.The vessels are so large and the meshes so small as to form analmost complete sinus. In the mid-dorsal and mid-ventral lines

NO. 310 S

256 MAURICE BLEAKLY

there are in the intestinal wall typhlosolar and sub-intestinaltracts. These tracts have no definite walls, but are rather longi-tudinal dilatations of the blood sinus, with which they communi-cate freely. Connecting the typhlosolar and sub-intestinal tractsare two pairs of circular tracts which encircle the gut in theanterior and posterior third of each segment.

These circular tracts are also mere specializations of the sinus.

D o r s a l Vesse l s (D.V.).

The dorsal vessel in 0 . t h o m a s i is represented by a pairof tubes running parallel to each other on either side of the mid-dorsal line. In the intestinal region the vessels lie close to thegut, and are covered by chloragogen cells continuous with thosesurrounding the tract.

Contrary to Beddard (4), who described the dorsal vesselas being 'completely' double in 0 . t h o m a s i , I find that fromthe posterior end to the region of the gizzard the two tubes aresegmentally connected by a definite transverse vessel. Each ofthe two dorsal vessels is slightly constricted as it passes throughthe septum, and at the constriction bears a pair of valves whichopen forwards, the nature of which will be described later. Theconnecting vessel, which is just anterior to the valves, is nota mere junction produced by the fusion of the two tubes buta short vessel of uniform diameter comparable with the rungof a ladder (fig. 5, PI. 8). The relation of the connecting vesselmay be shown beyond doubt by cutting one of the two dorsalvessels and emptying the blood from the other of the samesegment by pressing it back across the connective; the backpressure is sufficient to close the valves at the septal constriction,rendering it impossible to force the blood into a posterior segment.

The dorsal vessels are rythmically contractile and may beconsidered to consist of a series of segmental muscular chambers.Bach chamber has a slightly conical appearance, being dilatedposteriorly, and is marked off from those anterior and posteriorto it by the septal constrictions and their valves. Into eachchamber open two short dorso-intestinal vessels (fig. 2, PI. 8,D. int.) which join the circular tracts of the intestinal plexusat their union with the typhlosolar tract. In addition, at the

VASCULAR SYSTEM OF OCTOCHAETUS 257

anterior end of each segment just behind the septal constriction,each chamber receives a dorso-tegumentary vessel (D.teg.). Theopenings of the dorso-intestinals and dorso-tegumentaries areguarded by valves which project into the dorsal vessel directingthe blood into it. At the posterior end of the worm each of thetwo dorsal vessels arises in the last segment by the union ofcapillaries from the gut and from the integument, which maybe considered to represent the dorso-intestinal and dorse-tegu-rnentaries of this segment.

V e n t r a l Vessel (V.V.).The ventral vessel extends from the anterior to the posterior

end of the body lying in the mid-ventral line suspended bya mesentary from the gut. It is single throughout its length andis of uniform diameter.

In the intestinal region it gives off a pair of ventro-tegumen-tary and a single ventro-intestinal vessel in each segment. Theventro-tegumentaries leave the ventral-vessel just anteriorly tothe septal wall and run for a part of their course on the anteriorface of the septum. The ventro-intestinals arise from the vesselin the middle of each segment and run upward in the suspendingmesentary to join the sub-intestinal tract of the peri-intestinalplexus. At the posterior end of the body the ventral vesseldivides into branches distributed to the gut and integument.

There are no valves at any point in the course of the ventralvessel, which is non-contractile.

There is no s u b - n e u r a l vesse l in 0 . t h o m a s i or otherAcanthodrilid.

T e g u m e n t a r y Vessels (figs. 2, PI. 8, and 7, PI. 9).The d o r s o - t e g u m e n t a r y vessels (D.teg.) are paired and

lie against the posterior face of the septum in each segment.They are formed by tributaries from the body-wall and nephridiaand enter one or other of the dorsal vessels just posteriorly to theseptal constriction.

The v e n t r o - t e g u m e n t a r y vessels (V.teg.) are pairedvessels leaving the ventral vessel just anteriorly to the septum ineach segment. After running on the anterior face of the septumfor a short distance they pierce it and enter the next posterior

258 MAURICE BLEAKLY

segment. Here they ascend the septum circularly on its pos-terior face, and give off branches parallel to those which giverise to the dorso-tegumentaries.

There is no i n t e s t i n o - t e g u m e n t a r y system in thisregion.

Vascu l a r Sys tem in the A n t e r i o r TwentyS e g m e n t s .

A l i m e n t a r y P l exus and Assoc ia ted Vesse ls .

The alimentary plexus consisting of blood-channels lyingbetween the epithelial and muscular layers of the gut arisesin the seventh segment, i.e. directly behind the gizzard. Insegments 7 and 8 the plexus is represented by blood-channelslying in longitudinal folds of the epithelium. Posteriorly, thechannels form a closer network approximating to a sinus whichsurrounds the epithelium and is directly continuous with thesinus of the intestinal region. In segments 8-13 there is inaddition a more external network of capillaries lying beneaththe peritoneum but outside the muscular layer. These capil-laries communicate both with the internal plexus and with thelateral oesophageal and supra-intestinal vessels.

The s u p r a - i n t e s t i n a l vessel (S.I.V.) lies on the dorsalsurface of the oesophagus in segments 7 to 17 and communicatesin many places with the internal plexus of which it may beregarded to be a specialization. Posteriorly, it comes to an endin the sinus between the oesophageal gland in segment 17.Anteriorly, its branches give rise to capillaries distributed overthe gizzard. The vessel is single but bifurcates after passingthrough the posterior septum of each segment, the two branchescoalescing towards the middle of the segment. The loops thusformed are dilated in segments 10-13 and receive circular vesselsfrom the lateral oesophageal vessels. Coincident with the unionof these vessels is the point of origin of the intestinal portionof the latero-intestinal hearts (figs. 3, 6, PI. 8, and 9, PI. 9).

Dorsa l Vessel (D.V.).Anteriorly to the large intestine the two dorsal vessels leave

the surface of the tract though they are still bound to it by folds

VASCULAR SYSTEM OF OCTOCHAE.TUS 259

of the peritoneum and lie higher in the body-cavity. The doublenature of the vessel is retained up to segment G, the two tubesno longer remaining parallel but curving away from one anotherbetween the septa. In segment 6 slightly anteriorly to the con-necting vessel the two tubes coalesce, then separate, then coalesceagain to form a single vessel which continues forward to thefourth segment (fig. 4, PI. 8). In segment 4 over the pharyngealmass it divides into four pairs of vessels. These are:

(a) posteriorly, a pair of commissurals passing down throughthe tufted pepto-nephridia to join the ventral vessels (figs.1 and 4 A, PI. 8);

(b) two pairs which are distributed over the pharynx,one transversely, the other obliquely forward (figs. 1 and 4 B,PL 8); and

(c) most anteriorly, a pair which, after giving rise to smallbranches to the integument, continue forwards obliquely dorso-laterally to the cerebral commissure, beneath which they divide,the largest branch of each joining a corresponding branch fromthe ventral vessel, the smaller branches being distributed overthe wall of the buccal cavity (figs. 1 and 4 c, PI. 8).

In each of segments 14-20 the dorsal vessels receive a pairof dorso-intestinal vessels and in segments 18-20 in additiona pair of dorso-tegumentary vessels. These vessels are similarto those in the intestinal region.

There are four pairs of latero-intestinal hearts in segments10-13, two pairs of lateral hearts in segments 8 and 9, and a pairof commissurals in each segment anterior to this. These lastare connected with the dorsal vessel, but the openings to thedorsal vessel bear no valves.

V e n t r a l Vesse l .

The ventral vessel is continued forward to the second segment,where it bifurcates one branch going to each side of the buccalcavity, and after giving rise to small vessels to the integument,to the ventral surface of the pharynx, and to the walls of thebuccal cavity, joins the branch (c) from the dorsal vessel beneaththe circum-oesophageal commissure.

In segments 14-20 ventro-tegumentary and ventro-intestinal

260 MAURICE BLEAKLY

vessels similar to those in the intestinal region leave the ventralvessel. Anteriorly to segment 14 it receives a pair of hearts orcommissurals in each segment, but no vessels leave it beforeit bifurcates.

L a t e r a l - o e s o p h a g e a l Vesse ls (figs. 1, 8, and 4, PL 8).These are a pair of longitudinal vessels situated ventro-

laterally to the oesophagus in segments 4-13. In segments 8-13they are attached to the oesophagus and communicate with theperi-oesophageal plexus through numerous capillaries. Insegments anterior to this they become free from the gut andlie in the body-cavity, being the most prominent vessels of thisregion.

In the segments containing the latero-intestinal hearts(segments 10-13) the lateral-oesophageal vessels communicatewith the supra-intestinal by circular vessels in the wall ofthe oesophagus (fig. 3, PL 8), and may be considered to endposteriorly by joining the supra-intestinal by means of thesevessels. The lateral-oesophageal vessels cease in segment 4where they turn sharply upward and join to form a loop overthe crop. On either side this loop receives five vessels which ariseon the walls of the pharynx and buccal cavity (fig. 1, PL 8).

In each segment the lateral oesophageal vessels receiveintestino-tegumentary vessels, and may be considered to forma part of the intestine-tegumentary system, as Bourne (7) sug-gested, since they communicate both with the integument byway of the intestine-tegumentaries and with the peri-oesophagealplexus.

H e a r t s and C o m m i s s u r a l Vesse l s (figs. 1, 3, 6, PL 8,figs. 8, 9, PL 9).Posteriorly to segment 13 the dorsal and ventral vessels have

no direct communication with one another. But in segment 13,and in each segment anterior to it, there is a pair of hearts ora pair of commissural vessels connecting them. These con-necting vessels are of three types.

(a) The latero-intestinal hearts (L.I.H.) in segments 10-13communicate dorsally with both dorsal and supra-intestinalvessels and ventrally with the ventral vessel. Each heart con-

VASCULAR SYSTEM OF OCTOCHAETUS 261

sists of three contractile pear-shaped chambers and a smallventral spherical bulb, arranged in series (fig. 9, PI. 9). Thedorsal chamber, which is much larger than the others, is con-nected to the dorsal and supra-intestinal vessels, the openings ofwhich are guarded by valves opening towards the heart (fig. 10,PI. 9). The constriction between the lowest chamber and thebulb is guarded by a collar valve which directs the blood intothe ventral vessel with which the bulb communicates (figs. 3,6, PI. 8, figs. 9, 10, PI. 9).

(b) The l a t e r a l h e a r t s (L.H.) in segments 8 and 9 com-municate with dorsal and ventral vessels but not with the supra-intestinal vessels. They are in the form of a series of contractilebulbs (fig. 8, PI. 9). Near the origin from the dorsal vesselthere is a valve opening towards the heart, and a short distancefrom the ventral vessel there is a second valve opening ventrally,i.e. away from the heart. Between this valve and the ventralvessel there arises a ventro-tegumentary vessel.

(c) The c o m m i s s u r a l v e s s e l s , of which there are fivepairs, are non-contractile loops joining the dorsal and ventralvessels. In segments 5, 6, and 7 the commissurals bear at aboutone-third of its length from the dorsal vessel small bulbs, con-taining valves opening ventrally, above which no vessels leave.

Below this valve the commissurals in segment 7 give rise totwo tegumentary vessels (fig. 11, PI. 9); in segments 5 and 6the commissurals give rise to two vessels on each side, the dorsalpair to the gizzard, the ventral pair to the integument (fig. 1,PI. 8). The commissurals in segment 4 have no bulb or valves—they give rise to one vessel arising about half-way down andgoing to the pepto-nephridia and to another supplying theintegument (fig. 1).

The anterior branches (e) from the dorsal vessel, where itdivides above the pharynx, together with the branches from thebifurcation of the ventral vessel must be considered to be themost anterior pair of commissurals, since they unite to forma loop round the gut beneath the oesophageal nerve commissureand give rise to vessels with a distribution similar to those fromthe posterior commissurals, i.e. to the gut (pharynx and buccalcavity) and to the integument.

262 MAURICE BLEAKLY

T e g u m e n t a r y Vesse ls .

(a) I n t e s t i n o - t e g u m e n t a r y Vessels (fig. 1, PI. 8,Int.teg.).—In the cephalized region the body-wall is servedby a series of vessels connected with the intestinal vessels andnot with the dorsal vessels as is the case in the posterior regionof the body. The posterior pair of this series arises on the body-wall in segment 18 and continues forward on its ventro-lateralsurface to segment 13 or 14. Here the vessels pursue a transversecourse on the anterior face of the septa to within a short distanceof the gut, where they leave the septa and continue forward onthe oesophageal wall to join the lateral oesophageal vessels insegment 13. The actual segment (i.e. 13 or 14) in which thesevessels traverse the septa has been observed to vary in differ-ent specimens and even on either side of the same specimen.The remaining intestino-tegumentary vessels are segmentallyarranged and each is confined to one segment.

(b) V e n t r o - t e g u m e n t a r y Vesse ls (V.teg.).—The ven-tro-tegumentary vessels in segment 9 and in the segmentsanterior to this do not rise from the ventral vessel but from theventral portion of the lateral hearts or commissurals of thesesegments and each is confined to one segment.

The Va lves .

The presence of valves has been noted in the description ofthe dorsal vessels and of the hearts and commissurals.

The valves in 0 . t h o m a s i are of two types: (a) double,formed by two separate and opposed masses of tissue, and (b)circular or collar valve.

Double Valves (fig. 11, PI. 9) occur at the septal con-strictions of the dorsal vessels and also at the entrances of thedorso-tegumentaries and of the dorso-intestinals into thesevessels. They are in the form of pear-shaped lobes attached bytheir apices. When the valves are opened the lobes float freelyin the blood-stream, and when closed they fold back and meet,completely filling the aperture.

Ci rcu la r or Collar Valves (fig. 10, PI. 9) occur in thehearts and commissurals. They consist of a flap of tissue en-circling the vessel and attached by one edge to its inner wall,

VASCULAR SYSTEM OF OCTOCHAETUS 263

the other edge being free. When the valves are opened the flapshang freely in the blood-stream; when closed they form trans-verse partitions preventing the flow of blood.

The action of the valves can be studied in a recently killedworm. If pressure is applied to a vessel behind the valve itopens widely and the blood passes through. The slightestpressure applied to a vessel in front of a valve, however, causesthe valve to close and prevents the passage of blood through theopening. There seems to be no need to suppose that the actionof the valves is due to the muscular activity of the valves them-selves as Vejdovsky supposed (Stephenson (11)), but rather thatthere is the ' flap action' assumed by Stirling (Stephenson (11))which changes in blood-pressure are sufficient to explain.

THE COURSE TAKEN BY THE BLOOD.

All authorities are agreed that the blood flows forward in thedorsal vessel, downward in the hearts, and backward in theventral vessel behind the hearts in all earthworms, and thesimplest observation is sufficient to confirm these facts.

I n t e s t i n a l E e g i o n .

If the d o r s o - t e g u m e n t a r i e s are cut or broken bleedingoccurs from the distal end and not from the proximal end of thecut. Further, the valves which guard the entrances of the dorso-tegumentaries into the dorsal vessel allow the passage of bloodto, but not from, the dorsal vessel. It can therefore be definitelystated that the flow in the dorso-tegumentaries is into the dorsalvessel.

The d o r s o - i n t e s t i n a l s when severed do not bleed fromthe proximal end. Further the valves guarding their openingsinto the dorsal vessels close so as to prevent the blood passinginto the intestinal vessels.

Methyl violet introduced to the dorsal vessel could be tracedforward for some segments, but did not enter the tegumentaryor intestinal vessels.

Thus it is evident that in the intestinal region the dorsalvessels received blood from all vessels connected with them.

When the v e n t r o - t e g u m e n t a r i e s are severed they

264 MAURICE BLEAKLY

bleed freely from the ventral attachments but not at all fromtegumentary capillaries.

The v e n t r o - i n t e s t i n a l s when severed bleed slightly fromthe sub-intestinal end and profusely from that attached to theventral vessel, indicating that the pressure in the ventral vesselis much higher than in the intestinal vessels, and that the flowis from ventral vessel to the sub-intestinal tract in the intestinalplexus.

The flow in the tracts of the intestinal plexus is difficult todetermine since they contain large quantities of blood at fairlyeven pressure. When cut the tracts bleed freely from both endsand, since they are closely associated with the peri-intestinalplexus, clipping merely causes the blood to take an alternativeroute. However, since the blood enters the intestinal systemunder pressure from the ventral vessel, and is removed by thedorso-intestinals during diastole of the dorsal vessel, the flowmust be in general from ventral to dorsal round the gut.

In the intestinal region, therefore, blood passes from ventralto dorsal vessel by either of two routes, (1) through the ventro-tegumentaries to the nephridia and body-wall, through capillaryplexus in the skin for oxygenation and thence by dorso-tegu-mentaries to the dorsal vessels, or (2) through ventro-intestinalsto the gut-wall and thence by dorso-intestinals to the dorsalvessels.

The ventral vessel and the vessels that arise from it aretherefore anatomically arterial, supplying blood to the variousorgans; the dorsal vessels and vessels associated with them areanatomically venous, collecting blood and returning it to the heartsin the anterior cephalized region. It is to be noted that there canbe no complete segmental circulation such as Bourne (7) orHarrington (9) suggests since blood does not pass from dorsalto ventral vessels posteriorly to the hearts.

Cepha l ized Reg ion .

In the cephalized region the arrangement of the vessels differsfrom that posterior to it, and there is a corresponding differencein the course taken by the blood.

Anteriorly to segment 14 all vessels communicating with the

VASCULAR SYSTEM OF OCTOCHAETUS 265

dorsal vessels receive blood from them. This is evident from thereversed action of the valves in the hearts and commissurals.The anterior commissurals which bear no valves, when severedbleed from the dorsal portion indicating a flow of blood fromthe dorsal vessels. Anteriorly to segment 14 the ventral vesselcommunicates with no vessels save the hearts and commissurals.

The integument receives blood from the ventro-tegumentarieswhich posteriorly to segment 14 arise from the ventral vessel,anteriorly to segment 8 from the lateral hearts and commissurals,and between segments 8 and 14, from branches from these vesselscontinued forward or backward on the body-wall. Blood iscollected from the integument by the dorso-tegumentariesposterior to segment 18 and anteriorly to this by the intestino-tegumentaries which return it to the lateral-oesophageal vessel.The oesophagus receives blood from the ventro-tegumentaryposteriorly to segment 14 and anteriorly to this from the lateralvessel. The gizzard is supplied by vessels from the commissuralsin segments 5 and 6, the pharynx from branches of the dorsalvessel and also with the walls of the buccal cavity from theanterior commissural. Blood is collected from the oesophagusand gizzard by the supra-intestinal vessel, and from the pharynxand buccal cavity by the anterior branches of the lateraloesophageal vessels.

The lateral oesophageal vessels severed in segment 6 bleedfrom both ends, but more profusely from the anterior portion.Flow in the lateral vessel is therefore backward to the oesophagusand circular vessels.

Flow in the supra-intestinal is either forward or backward tothe hearts in the region of which there is a reduction of pressureat each diastole.

Flow in the commissurals is towards the ventro-tegumentaryvessels. In the portion which lies between these and the ventralvessel it is more difficult to determine. When cut this portionusually bled freely from both ends; in three specimens, however,the bleeding was distinctly more profuse from the ventralportion, which indicated that the flow was towards the ventro-tegumentary from the ventral vessel.

The ventral vessel when severed some distance anteriorly to

266 MAURICE BLEAKLY

the hearts bled freely from both ends indicating a fairly evenpressure within the vessel. When clipped at about the samepoint, no dilatation could be detected. When the ventral vesselwas clipped nearer the heart it became dilated posteriorly ateach contraction of the heart, the dilatation becoming reducedbetween contractions. The blood did not drain from the vesselbetween contractions of the heart as Combault stated to bethe case.

Blood enters the ventral vessel from the heart at each con-traction, and flows forward in front of and backward behindthe hearts. Of the blood entering the anterior portion of theventral vessel a part appears to be tidal and flows back past thehearts as the pressure is reduced between contractions, anda part continues forward and leaves the vessel through the ven-tral portion of the commissurals. As further evidence of theforward flow in the ventral vessel it may be noted that thepaired lateral vessels are of much greater capacity than eitherthe dorsal or ventral vessels of this region. It is evident, there-fore, that the lateral oesophageal vessels are returning much moreblood from the anterior region than the dorsal vessel alone couldsupply, and the extra blood can come only from the ventral vessel.

Fig. 12, PI. 9, gives a diagrammatic representation of thecourse of the circulation in both intestinal and cephalized regions.

COMPARISON WITH OTHER TYPES.

The vascular system which has been described agrees in allessential respects in regard to the distribution of similar vesselsand general scheme of circulation with the accounts of recentauthors.

It agrees with L u m b r i c u s in that there is no secondarysystem of vessels supplying the intestine either directly fromthe integument as occurs in M e g a s c o l e x , or indirectly fromthe integument through septo-intestinals from sub-neural-dorsal commissurals as is the case in P h e r e t i m a .

In the cephalized region no vessels supply blood directlyfrom the longitudinal trunks to the integument in the segmentscontaining the hearts. In this respect L u m b r i c u s andMegasco lex are similar whilst P h e r e t i m a is different.

VASCULAR SYSTEM OF OCTOCHAETUS 267

The absence of vessels to the integument arising directly fromthe ventral vessel in the region anterior to the heart is notparalleled in other types. However, if the commissurals wereincomplete and the branch from the dorsal vessels supplied thegut, and the ventral vessel the ventro-tegumentaries, a systemwould be obtained almost exactly similar to that in P h e r e -t i m a .

The circular vessels which put the lateral oesophageal vesselsin communication with the supra-intestinal at the origin of thelater-intestinal hearts from the latter, form an intermediatecondition between the relations of the lateral oesophagealvessels in P h e r e t i m a where they communicate with thesupra-intestinal and thence with the hearts, and in D a r w i d i awhere the circular vessels communicate directly with the hearts.

SUMMARY.

1. Posteriorly to the gizzard the dorsal vessel of 0 . t h o m a s iis double, consisting of two tubes joined by a short connectingvessel anterior to the septum in each segment.

2. There are six pairs of strongly contractile hearts in seg-ments 8-13. The latero-intestinal hearts in segments 10-13receive blood from the dorsal vessel and from a common trunkfrom the supra-intestinal and latero-oesophageal vessels, andsupply blood to the ventral vessel. The lateral hearts receiveblood from the dorsal vessel and supply ventro-tegumentaryand ventral vessels. For complete circulation all blood mustpass through the hearts.

3. The ventral vessel, which is not contractile, receives bloodfrom the hearts; posteriorly to the hearts the flow is backwardin the ventral vessel and outward via ventro-tegumentaries andventro-intestinals to the body and gut; anteriorly to the heartsthe flow is forward, the blood leaving the ventral vessel by thecommissures from which the ventro-tegumentaries of theanterior region arise. The ventral vessel is the main arterialtrunk.

4. Posteriorly to the hearts the dorsal vessel collects blood fromthe body and gut by dorso-tegumentaries and dorso-intestinals.The flow in the dorsal vessel, which is contractile, is forward.

268 MAURICE BLEAKLY

Blood leaves the dorsal vessel through small vessels to the latero-intestinal hearts, and larger vessels to the lateral hearts. An-teriorly to the hearts, the commissures and all vessels connectedwith the dorsal vessel receive blood from it. Thus, posteriorlyto the hearts the dorsal vessel is venous, and anteriorly to thehearts arterial, in character.

5. Blood from the anterior region is returned by paired latero-oesophageal vessels to the supra-intestinal vessel and to thelatero-intestinal hearts. The latero-oesophageal vessels and thesupra-intestinal vessels are the main venous trunks in theanterior region.

6. There is no sub-neural vessel.

BIBLIOGRAPHY.

1. Bahl, K. N. (1919).—"New Type of Nephridia in Indian Earthwormsof the Genus Pheretima", 'Quart. Journ. Micr. Sci.', vol. 64.

2. (1921).—" Blood-Vascular System of the Earthworm Pheretimaand Course of Circulation in Earthworms", ibid., vol. 65, pp. 350-92.

3. Beddard, F. E. (1885).—"Specific Characters of Structure of CertainNew Zealand Earthworms", 'P.Z.S.', pp. 810-32.

4. (1892).—"On Some New Species of Earthworms from VariousParts of the World", ibid., pp. 666-706.

5. Benham, W. Blaxland (1891).—"The Nephridium of Lumbricus andits Blood Supply", 'Quart. Journ. Micr. Sci.', vol. 36.

6. (1900).—"An Account of Acanthodrilus (Maoridrilus) uliginosus(Hutton)", 'Trans. N.Z. Inst.', vol. 33, p. 122.

7. Bourne, A. G. (1891).—"On Megascolex coeruleus and a Theory ofCourse of Blood in Earthworms", 'Quart. Journ. Micr. Sci.', vol. 32.

8. (1894).—"On Moniligaster grandis", ibid., vol. 36.9. Harrington, N. R. (1899).—"Calciferous Glands of Earthworms with

Appendix on the Circulation", 'Journ. Morphol.', vol. xv.10. Johnstone, J. B., and Johnstone, S. W. (1902).—" Course of Blood Flow

in Lumbricus", 'American Naturalist', vol. 36.11. Stephenson, J. (1930).—'The Oligochaeta.' Oxford.

VASCULAK SYSTEM OF OOTOCHAETUS 269

EXPLANATION OF PLATES 8 AND 9.

LETTERING.

Hearts and commissures—untouched vessels.Arterial vessels, i.e. vessels carrying blood from hearts and supplying

body and gut—banded vessels.Venous vessels, i.e. vessels returning blood to hearts:

Dorsal vessel—cross hatched.Other vessels—black.

a., b., and c, anterior branches of dorsal vessel; Buc, Buccal cavity;com., commissural vessel; con., vessel connecting dorsal vessels; Cr., Crop;c.t., circular tract; D.int., Dorso-intestinal; D.teg., Dorso-tegumentary;D.V., Dorsal vessel; Giz., Gizzard; Int., Intestine; int.teg., intestino-tegumentary; i.p., internal plexus; L.H., Lateral heart; L.I.H., Latero-intestinal heart; L.V., Latero-oesophageal vessel; N., Pepto-nephridia;ox., circum-oesophageal commissure; Oes., oesophagus; Oes.Ol., Oeso-phageal gland; S.I.V., supra-intestinal vessel; Sept., septum; Sub.int.,Sub-intestinal; t.t., typhlosolar tract; V.int., Ventro intestinal; V.teg.,Ventro-tegumentary; V.V., Ventral vessel.

PLATE 8.

Pig. 1.—Lateral view of gut and blood-vessels in segments 1-20. Thesepta are indicated by the short vertical lines above and the segments byRoman numerals.

Fig. 2.—Lateral view of a segment in the intestinal region with body-wallcut away above and below leaving the septa free.

Kg. 3.—Lateral view of segment 12 to show the relations of the latero-intestinal parts to the vessels of that segment.

Fig. 4.—Dorsal view of segments 1-8.Kg. 5.—Dorsal view of dorsal vessel in intestinal region.Kg. 6.—Dorsal view of segment 12 to show relations of dorsal and supra-

intestinal vessels and the latero-intestinal hearts.

PLATE 9.

Fig. 7.—Composite transverse section in the intestinal region:To the left: through a circular tract showing septum and tegumentaryvessels.To the right: septum incomplete ventrally to show the origin of theventro-tegumentary vessel in the preceding segment.

Fig. 8.—Composite transverse section through segments 7 (right) and8 (left), to show in one the commisaural and the other the lateral hearts.The origin of the tegumentary vessels is shown.

Fig. 9.—Transverse section through segment 12 showing latero-intestinalhearts, origin of tegumentary vessels, circular tracts, and inner plexus.

270 MAURICE BLEAKLY

Fig. 10.—Diagram to show the action of the collar valves in the latero-intestinal hearts in diastole (left) and systole (right).

Fig. 11.—Diagram of the double valve in the region of a septal constric-tion of the dorsal vessels. Chambers X and Y' diastole with valves guardingthe openings of D.teg. and D.int. vessels and posterior septal constrictionopen and valves guarding anterior septal constriction open. Chambers X'and Y systole.

Fig. 12.—Tabular diagram to illustrate the course taken by the bloodin the intestinal and cephalized regions.

Quart. Journ. Micr. Sci. Vol. 78, N. S., PI S

Bleakly

Quart. Journ. Micr. Sri. Vol. 78, N. S., PI 9

D.v.

nt.D beg. 11

Dorsal Vessel-

D.int.

->Dorsal VesseL

D.t?9

Typh

I .Int. plexus c.v.

PjTarynx

Sub. int.'

V.teg.

V. i n t

Ventral Vessel-<- - Ventral Vessel

INTESTINAL REGION CEPHALISED REGION

Bleakly