Invertebrates 2 Ctenophora, Platyhelminthes, Nematoda

Lecture outline 1.Phylum Ctenophora (briefly) 2.Phylum Platyhelminthes 3.Phylum Nematoda

Phylogenetic overview (cont.)

Phylum Ctenophora: The comb jellies  Along with Cnidarians, they are radially symmetrical

Phylum Platyhelminthes: The flatworms

Evolutionary position: Development of a third germ layer and bilateral symmetry  Cnidarians, Ctenophores All others  Radial symmetry, diploblastic Bilateral symmetry, triploblastic  (Porifera)

Still no body cavity!  Platyhelminthes All others have body cavities  Porifera Cnidaria  Ctenophora

Basic Platyhelminthes features  Eversible pharynx  Gastrovascular cavity  Structure  Functions  Water and ion balance  Nervous system  Unidirectional AP  brain  distinct nerve cords sensory organs  Movement

Reproduction  Hermaphrodites  Internal fertilization  penis, sometimes with stylet  hypodermic impregnation  stab prey  R-rated flick…

 The blood fluke Schistosoma infects 200 million people, leading to body pains, anemia, and dysentery. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig

 Tapeworms (class Cestoidea) are also parasitic.  The adults live mostly in vertebrates, including humans.  Anterior suckers and hooks anchor the worm in the digestive tract of the host.  A long series of proglottids, sacs of sex organs lie posterior to the scolex.  Tapeworms absorb food particles from their hosts. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig

Phylum Nematoda: The roundworms

Evolutionary features: Body cavity!  Platyhelminthes Nematoda and all others!  Porifera Cnidaria  Ctenophora

Nematode body cavity: pseudocoelom  Mesoderm on inside of body wall, but not surrounding gut

Body plan  Feeding/digestive system:  Relatively simple (not highly branched) gut, but it is complete, with anus!  Gas exchange  Body surface  Coelomic cavity  No circulatory system  Excretory system/osmotic regulation  Exists, not well-understood  Locomotion:  Have only longitudinal muscles  Demo: The nematode swim!

Body plan (cont.)  Nervous system  Four nerve cords: 1 dorsal, 1 ventral, and 2 lateral  ganglia along ventral cord (will see this in arthropods/annelids)  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