Fig. 14.10

ACOELOMATE ACOELOMATE ANIMALS ANIMALS

• 14.1. General 14.1. General Features Features

• A. Cephalization A. Cephalization • 1. Sessile animals 1. Sessile animals

survive well with survive well with radial symmetry. radial symmetry.

• 2. Concentrating 2. Concentrating the sense organs on the sense organs on the head is an the head is an advantage to active advantage to active animals that seek animals that seek food; this provides food; this provides an anterior and an anterior and posterior end and posterior end and bilateral symmetry.bilateral symmetry.

• B. Position and Biological B. Position and Biological Contributions Contributions

• 1. These are the simplest animals with 1. These are the simplest animals with primary bilateral symmetry. primary bilateral symmetry.

• 2. They have a solid body 2. They have a solid body without a without a coelomcoelom; they are the acoelomate ; they are the acoelomate bilateria. bilateria.

• 3. Specialization of organs provides the 3. Specialization of organs provides the organ-system level of organizationorgan-system level of organization. .

• 4. They are protostomes with spiral 4. They are protostomes with spiral cleavage; flatworms and nemerteans cleavage; flatworms and nemerteans have determinate cleavage. have determinate cleavage.

• 5. The mesoderm is well-defined; they 5. The mesoderm is well-defined; they are are triploblastictriploblastic with all three germ with all three germ layers. layers.

• 6. Along with cephalization, this is the 6. Along with cephalization, this is the beginning of a ladder-type of nervous beginning of a ladder-type of nervous system. system.

• 7. The simplest excretory or 7. The simplest excretory or osmoregulatory systems appear. osmoregulatory systems appear.

• 8. Nemerteans have the simplest 8. Nemerteans have the simplest circulatory systemcirculatory system and a one-way and a one-way alimentary canal with alimentary canal with both mouth and both mouth and anusanus. .

• 14.2. Phylum 14.2. Phylum Platyhelminthes Platyhelminthes

• A. Characteristics A. Characteristics • 1. A cellular 1. A cellular mesodermal mesodermal

parenchyma parenchyma replaced the gelatinous replaced the gelatinous mesoglea. mesoglea.

• 2. In some platyhelminths, 2. In some platyhelminths, parenchyma is made of non-contractile parenchyma is made of non-contractile cell bodies of muscle cells connected cell bodies of muscle cells connected in somewhat the same manner as in somewhat the same manner as epitheliomuscular cells of cnidarians. epitheliomuscular cells of cnidarians. (Fig. 13-7) (Fig. 13-7)

• 3. Flatworms vary from a millimeter to 3. Flatworms vary from a millimeter to many meters in length. many meters in length.

• 4. Some flatworms are free-living; 4. Some flatworms are free-living; others are parasitic. others are parasitic.

• 5. Turbellarians are mostly free-living 5. Turbellarians are mostly free-living in aquatic or moist terrestrial in aquatic or moist terrestrial environments; some are symbiotic or environments; some are symbiotic or parasitic. parasitic.

• 6. All flukes and tapeworms are 6. All flukes and tapeworms are parasitic. parasitic.

• 7. Parasitic flatworms may have 7. Parasitic flatworms may have several hosts in their life cycle; some several hosts in their life cycle; some larvae may be free-living. larvae may be free-living.

• B. Form and Function B. Form and Function • 1. Tegument and Muscles 1. Tegument and Muscles

a. Most turbellarians have a a. Most turbellarians have a cellular, ciliated epidermis on a cellular, ciliated epidermis on a basement membrane. basement membrane.

b. Rod-shaped rhabdites swell b. Rod-shaped rhabdites swell and form a protective mucous and form a protective mucous sheath. (Fig. 14-3) sheath. (Fig. 14-3)

c. Most turbellarians have dual-c. Most turbellarians have dual-gland adhesive organs. (Fig. 14-gland adhesive organs. (Fig. 14-4) 4)

» 1) 1) Viscid gland cells Viscid gland cells fasten microvilli of fasten microvilli of anchor anchor cells cells to the substrate. to the substrate.

» 2) Secretions of 2) Secretions of releasing releasing gland cells gland cells provide a provide a quick chemical quick chemical detachment. detachment.

d. Under the basement d. Under the basement membrane, muscle fibers run membrane, muscle fibers run circularly, longitudinally and circularly, longitudinally and diagonally. diagonally.

• 2. Nutrition and 2. Nutrition and DigestionDigestion

a. Cestodes have no digestive a. Cestodes have no digestive system; others have a mouth, system; others have a mouth, pharynx and an intestine. (Fig. pharynx and an intestine. (Fig. 14-6) 14-6)

b. In planarians, the pharynx b. In planarians, the pharynx can extend through the mouth can extend through the mouth that is mid-ventrally located. that is mid-ventrally located.

c. The intestine has three c. The intestine has three branches, one anterior and branches, one anterior and two posterior. two posterior.

d. This gastrovascular cavity is d. This gastrovascular cavity is lined with columnar lined with columnar epithelium. epithelium.

e. The mouth of trematodes e. The mouth of trematodes and monogeneans opens near and monogeneans opens near the anterior end; the pharynx the anterior end; the pharynx is not extensible; the intestine is not extensible; the intestine ends blindly and varies in the ends blindly and varies in the degree of branching. (Figs. 14-degree of branching. (Figs. 14-7 and 14-16) 7 and 14-16)

• f. Planaria f. Planaria FeedingFeeding

1) They are 1) They are carnivorous and carnivorous and detect food by detect food by chemoreceptors. chemoreceptors.

2) They entangle 2) They entangle food in mucous food in mucous secretions from secretions from glands and glands and rhabdites. rhabdites.

3) They wrap 3) They wrap themselves around themselves around prey and extend the prey and extend the proboscis to suck up proboscis to suck up bits of food. bits of food.

• 3. Excretion and Osmoregulation 3. Excretion and Osmoregulation

a. Except in order Acoela, flatworms have a. Except in order Acoela, flatworms have protonephridia protonephridia with with flame flame cellscells. (Fig. 14-6) . (Fig. 14-6)

b. Beating flagella drive fluids down collecting ducts, forming a negative b. Beating flagella drive fluids down collecting ducts, forming a negative pressure drawing fluids through a network or weir. pressure drawing fluids through a network or weir.

c. The wall of the duct beyond the flame cell bears folds or microvilli to c. The wall of the duct beyond the flame cell bears folds or microvilli to resorb certain ions and molecules. resorb certain ions and molecules.

• 4. Nervous 4. Nervous System System

a. The subepidermal a. The subepidermal nerve plexus nerve plexus resembles the nerve resembles the nerve net of cnidarians. net of cnidarians.

b. One to five pairs b. One to five pairs of longitudinal nerve of longitudinal nerve cords lie under the cords lie under the muscle layer. muscle layer.

c. More derived c. More derived flatworms have flatworms have fewerfewer nerve cords. nerve cords.

d. Freshwater d. Freshwater planarians have one planarians have one ventral pair of nerve ventral pair of nerve cords forming a cords forming a ladder-type pattern; ladder-type pattern; the brain is a bilobed the brain is a bilobed ganglion anterior to ganglion anterior to the ventral nerve the ventral nerve cords. cords.

e. Except in acoels, e. Except in acoels, nerves are now nerves are now sensory, motor and sensory, motor and association-types. association-types.

• 5. Sense 5. Sense Organs Organs

a. a. Active Active locomotion locomotion favored favored cephalization cephalization and evolution of and evolution of sense organssense organs. .

b. b. OcelliOcelli are are light-sensitive light-sensitive eyespots in eyespots in turbellarians, turbellarians, monogeneans monogeneans and larval and larval trematodes. trematodes.

c. Sensory nerve c. Sensory nerve endings are in endings are in oral suckers, oral suckers, holdfast organs holdfast organs and genital and genital pores of pores of parasitic groups. parasitic groups.

• 6. Reproduction and Regeneration6. Reproduction and Regeneration

a. Fission a. Fission

» 1) Many turbellarians 1) Many turbellarians constrict behind constrict behind the pharynx and separate into two the pharynx and separate into two animalsanimals. .

» 2) Each half regenerates the missing 2) Each half regenerates the missing parts; this provides for rapid population parts; this provides for rapid population growth. growth.

» 3) Some do not separate immediately, 3) Some do not separate immediately, creating chains of zooids. (Fig. 14-8) creating chains of zooids. (Fig. 14-8)

b. Some asexual reproduction occurs in b. Some asexual reproduction occurs in intermediate hosts; see life cycles to follow. intermediate hosts; see life cycles to follow.

c. Nearly all are c. Nearly all are monoeciousmonoecious but cross- but cross-fertilize. fertilize.

d. Endolecithal eggs with spiral determinate d. Endolecithal eggs with spiral determinate cleavage are typical and ancestral. cleavage are typical and ancestral.

e. Some turbellarians and all other groups e. Some turbellarians and all other groups have female gametes with little yolk; yolk is have female gametes with little yolk; yolk is contributed by separate organs, contributed by separate organs, vitellariavitellaria. .

f. Male Structures f. Male Structures

» 1) One or more testes are connected to 1) One or more testes are connected to vasa efferentia that connect to one vas vasa efferentia that connect to one vas deferens. deferens.

» 2) The vas deferens runs to a seminal 2) The vas deferens runs to a seminal vesicle. vesicle.

» 3) A 3) A papilla-like penis or extensible papilla-like penis or extensible cirrus is the copulatory organcirrus is the copulatory organ. .

• C. C. Class Turbellaria Class Turbellaria • 1. This class includes the planaria.1. This class includes the planaria.

• 2. Turbellarians are mostly free-2. Turbellarians are mostly free-living and range from 5 mm to 50 living and range from 5 mm to 50 cm long. cm long.

• 3. Very small planaria swim by 3. Very small planaria swim by cilia. cilia.

• 4. Others move by cilia and 4. Others move by cilia and gliding over a slime track secreted gliding over a slime track secreted by marginal adhesive glands. by marginal adhesive glands.

• D. Class Trematoda • 1. All trematodes are

parasitic flukes.

• 2. Most adults are endoparasites of vertebrates.

• 3. They resemble ectolecithal turbellaria but the tegument lacks cilia in adults.

• 4. Adaptations for parasitism include:

a. penetration glands,

b. glands to produce cyst material,

c. hooks and suckers for adhesion, and

d. increased reproductive capacity.

• 5. Sense organs are poorly developed.

• F. F. Class Cestoda Class Cestoda • 1. Tapeworms have a 1. Tapeworms have a

unique flattened and unique flattened and segmented shape segmented shape compared to other compared to other flatworms. flatworms.

• 2. Each trailing segment is 2. Each trailing segment is a a proglottid proglottid containing a containing a set of reproductive organs. set of reproductive organs.

• 3. The entire surface of 3. The entire surface of cestodes is covered with cestodes is covered with projections similar to projections similar to microvilli seen in the microvilli seen in the vertebrate small intestine; vertebrate small intestine; these these microtriches microtriches increase the surface area increase the surface area for food absorptionfor food absorption. .

• 4. Nearly all are 4. Nearly all are monoecious. monoecious.

• 5. They lack sensory 5. They lack sensory organs except for modified organs except for modified cilia. (Fig. 14-17) cilia. (Fig. 14-17)

• 6. The 6. The scolex scolex is a holdfast is a holdfast head portion with suckers head portion with suckers and hooks. and hooks.

• 14.3. Phylum Nemertea

• A. Characteristics • 1. Nemerteans are

often called ribbon worms; an alternative phylum name is Rhynchocoela.

• 2. They are the simplest animals with a blood-vascular system.