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Noncoelomate Invertebrates

Chapter 32

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Invertebrate Phylogeny

• Two Approaches– Traditional reconstructions are based on

key aspects of body architecture. lumping phyla that share fundamental

aspects of body plan– New reconstructions employ molecular

comparisons. focus on differences in ribosomal RNA

sequences

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Traditional Protostome Phylogeny

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Novel rRNA Protostome Phylogenies

• Two major clades– Lophotrochozoans

flatworms mollusks annelids

– Ecdysozoans roundworms arthropods

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rRNA Protostome Phylogeny

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Parazoa

• Sponges (Porifera)– most lack symmetry– adults are sessile– little coordination among cells– three functional layers

choanocytes mesohyl outer epithelial layer

spicules

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Parazoa

• Beating of flagella lining interior draws water in through numerous pores.

– Small organisms are filtered out of the water, which flows through passageways and eventually out an osculum.

• Reproduction done by fragmentation as well as sexually.

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Sponges

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Radiata

• Distinct tissues– epidermis and nervous system develop

from ectoderm– gastrodermis develops from endodermis

• True body symmetry– Radiata

Cnidaria Ctenophora

– Bilateria all others

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Radiata

• Cnidarians– nearly all marine– carnivorous

cnidocytesnematocysts

– two basic body plans polyps and medusae

– fertilized eggs give rise to planulae– internal extracellular digestion

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Two Body Forms

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Cnidarians

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Radiata

– Classes of Cnidarians Hydrozoa - hydroids Scyphozoa - jellyfish Cubozoa - box jellyfish Anthozoa - sea anemones and corals

• Ctenophorans (comb jellies)– propel through the water by means of

eight comb-like plates of fused cilia

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Bilateral Acoelomates

• Bilateral symmetry– digestive tract is only internal cavity

• Phylum Platyhelminthes: the flatworms– Flatworms are among the simplest

bilaterally symmetrical animals, but they have a definite head at the anterior end, and do possess organs.

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The Bilateral Acoelomates

• Flatworms– cannot feed, digest, and eliminate food

simultaneously– absorb food directly through body walls– have excretory system

flame cells– most are hermaphroditic

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Flatworm Architecture

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The Bilateral Acoelomates

– Class Turbellaria: turbellarians only one of three classes are freeliving

– Class Trematoda: flukes life cycle liver fluke Clonorchis sinensis

miracidium - eggs (passed in feces) rediae - elongated, nonciliated larvaecercariae - tadpole-like larval stagemetacercariae - adult stage

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The Bilateral Acoelomates

– Class Cestoda: tapeworms hang on to inner walls of their hosts by

specialized terminal attachment organs, and absorb food through their skins.

scolex - attachment organneck - unsegmentedproglottids - repetitive segments

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The Bilateral Acoelomates

• Phylum Nemertea: ribbon worms– simplest animals that possess a complete

digestive system mouth and anus

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The Pseudocoelomates

• Internal body cavity– pseudocoel serves as hydrostatic skeleton

gains rigidity from being filled with fluid under pressure

– lack a defined circulatory system

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The Pseudocoelomates

• Phylum Nematoda: roundworms– bilaterally symmetrical unsegmented worms

covered by flexible, thick cuticle– mouth equipped with stylets– food passes through mouth as result of

sucking action of pharynx– lack flagella or cilia– reproduction is sexual– Trichinella regularly parasitize humans

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Roundworms

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Nematode-Caused Diseases

• Trichinosis - Trichinella• Pinworms - Enterobius• Intestinal roundworms - Ascaris• Filariasis - Filaria

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The Pseudocoelomates

• Phylum Rotifera: rotifers– small, bilaterally symmetrical, basically

aquatic animals with a crown of thick cilia at their heads

– well developed food processing apparatus– “wheel animals”

• Phylum Cycliophora– relatively new

circular mouth surrounded by fine, hairlike cilia