Bi 212: Lecture 3Bi 212: Lecture 3

Platyhelminthes, Nematoda, Annelida

Phylum Platyhelminthes:Phylum Platyhelminthes:The flatwormsThe flatworms

Require an aquatic habitat Often under rocks

Some free-living, but many parasitic forms Hosts provide

aqueous environment

Evolutionary relationshipsEvolutionary relationships

Platyhelminthes are Platyhelminthes are triploblastic, but have no body triploblastic, but have no body

cavitycavity

Note positions of embryonic tissue layers (section is not through pharynx…)

Protostomes vs. deuterostomesProtostomes vs. deuterostomes

Basic Platyhelminthes featuresBasic Platyhelminthes features

Eversible pharynx How does it work?

Gastrovascular cavity Functions

Water and ion balance

Protonephridia (flame bulb system)

Nervous system Brain? Nerve cords?

Sensory structures?

Movement Muscles Cilia and mucus

ReproductionReproduction

Hermaphrodites Internal fertilization

penis, sometimes with stylet hypodermic impregnation (R-rated flick) Penis is also used for???

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 33.12

Tapeworms are one many types of parasitic Platyhelminthes

Phylum Nematoda:Phylum Nematoda:The roundwormsThe roundworms

Covered with cuticle like Arthropoda Contains chitin Molts cuticle as it grows

Most abundant phylum in terms of numbers (?)

Found in nearly every environment

Some free-living, but also important parasites on plants/animals

Evolutionary relationshipsEvolutionary relationships

Nematode body cavity: Nematode body cavity: pseudocoelompseudocoelom

Mesoderm on inside of body wall, but not surrounding gut (thus not a true coelom or “eucoelom”)

Roles of pseudocoelom?

Body planBody plan Feeding/digestive system

Nematodes suck! Complete digestive system

Gas exchange Body surface Intestinal surface (minimal) Body cavity (pseudocoelom) No circulatory system

Excretory system/osmotic regulation “lateral line”, not well understood

Locomotion: Have only longitudinal muscles

Demo: The nematode swim! Elastic cuticle

Body plan (cont.)Body plan (cont.)

Nervous system “Brain”: nerve ring Four nerve cords (where?) Ganglia along ventral cord

What are ganglia?

Unique connection of muscles and nerves What is unique about them?

Sensory structures

Reproduction, etc…Reproduction, etc…

Reproduction Sexes usually separate, but some

hermaphrodites Internal fertilization

Many parasitic forms Trichina worms Heartworms River blindness Elephantiasis Also parasitic in plants

Phylum Annelida:Phylum Annelida:The segmented wormsThe segmented worms

Evolutionary relationshipsEvolutionary relationships

Acoelomates, Pseudocoelomates Acoelomates, Pseudocoelomates and Coelomatesand Coelomates

Pseudocoelom

Coelom

Surface area for nutrient absorption and gas exchange

+ +

Larger organs + +

Fluid cushions/ protects organs

+ +

Hydrostatic skeleton is possible

+ +

Completely lined by mesoderm

- +

Mesenteries suspend internal organs

- +

Independent movement of gut

- +

Basic Annelida featuresBasic Annelida features

Have all features of protostomes (know!)

Have body segmentation (new feature) Repetition of body parts Similar structures found in each region Some specialization of regions Some continuous parts (digestive

system)

Annelid diversityAnnelid diversity

Class Polychaeta

Class Oligochaeta

Class Hirudinea

Body PlanBody Plan Digestive

system Complete Specialized regions

Circulation Closed system Multiple hearts

Gas exchange Body surface Coelom *Parapodia

(Polychaetes only) Excretory

Metanephridia

Body Plan (cont.)Body Plan (cont.) Nervous system

Brain (=cerebral ganglia)

Ventral nerve cord (fused pair of cords)

One ganglion per segment

Movement Peristalsis Setae

Feeding in different classesFeeding in different classes Class Oligochaeta: Earthworms

Earth suckers (create vacuum by expanding muscular pharynx)

Why important in ecosystem? Class Polychaeta: Marine segmented

worms Many are tentacle feeders Many other feeding modes

Class Hirudinea: Leeches Food suckers (create vacuum by expanding

pharynx, digestive tract) How do blood sucking leeches feed?. What three substances do blood sucking leeches

secrete to aid in the process? NOTE: Not all leeches are blood suckers!