Bi 212: Lecture 3 Platyhelminthes, Nematoda, Annelida

Phylum Platyhelminthes: The flatworms   Require an aquatic habitat  Often under rocks   Some free- living, but many parasitic forms  Hosts provide aqueous environment

Evolutionary relationships

Platyhelminthes are triploblastic, but have no body cavity  Note positions of embryonic tissue layers (section is not through pharynx…)

Protostomes vs. deuterostomes

Basic 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

Reproduction  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  Tapeworms are one many types of parasitic Platyhelminthes

Phylum Nematoda: The 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 relationships

Nematode body cavity: pseudocoelom  Mesoderm on inside of body wall, but not surrounding gut (thus not a true coelom or “eucoelom”)  Roles of pseudocoelom?

Body 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.)  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  Sexes usually separate, but some hermaphrodites  Internal fertilization  Many parasitic forms  Trichina worms  Heartworms  River blindness  Elephantiasis  Also parasitic in plants

Phylum Annelida: The segmented worms

Evolutionary relationships

Acoelomates, Pseudocoelomates and Coelomates

PseudocoelomCoelom  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 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 diversity Class Polychaeta Class Oligochaeta Class Hirudinea

Body Plan   Digestive system  Complete  Specialized regions   Circulation  Closed system  Multiple hearts   Gas exchange  Body surface  Coelom  *Parapodia (Polychaetes only)   Excretory  Metanephridia

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 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!