32.1 The classification of invertebrates is currently being reevaluated.
An Uproar Over Invertebrate Phylogeny
• Great disagreement exists as to how the 35 animal phyla are related. (p. 634)
• Animal family trees have historically been based on lumping key aspects of body architecture, and more recently by focusing on differences in ribosomal RNA sequences. (p. 634)
• In the traditional animal family tree, bilaterally symmetrical animals are divided into acoelomates, pseudocoelomates, and coelomates. Phylogenies based on rRNA sequences recognize two clades: lophotrochozoans and ecdysozoans. (pp. 634-635)
32.2 The simplest animals are not bilaterally symmetrical.
Parazoa
• Sponges are multicellular animals that lack tissue-level development and body symmetry, and uniquely contain choanocytes with flagella, which line the internal cavity of the sponge. The beating of these flagella draws water in and through the sponge, thus bringing in food and oxygen and expelling wastes. (p. 636)
Radiata
• Distinct tissues are seen in the eumetazoans. Different cell layers in the embryo form different tissues and give rise to the basic body plan. (p. 638)
• Cnidarians are gelatinous organisms made of distinct tissues but lacking true organs. They have two basic body forms -- polyps and medusae -- and contain specialized cells called cnidocytes on their tentacles. (p. 638)
• There are four basic classes of cnidarians: Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa. (pp. 640-641)
• Ctenophores propel themselves through the water with fused cilia. (p. 641)
32.3 Acoelomates are solid worms that lack a body cavity.
The Bilaterian Acoelomates
• Bilateria are characterized by bilateral symmetry, allowing high levels of specialization. (p. 642)
• Phylum Platyhelminthes, the flatworms, contains class Turbellaria, (Turbellarians), class Trematoda (flukes), and class Cestoda (tapeworms). (pp. 642-645)
• Phylum Nemertea contains the ribbon worms. (p. 645)
32.4 Pseudocoelomates have a simple body cavity.
The Pseudocoelomates
• In the pseudocoelomates, circulation occurs in a pseudocoel, not in a defined circulatory system. (p. 646)
• Phylum Nematoda contains roundworms, which are bilaterally symmetrical and unsegmented. (p. 646)
• Many diseases are caused by nematodes, such as Trichinella, Enterobius, Ascaris, and Filaria. (p. 647)
• Phylum Rotifera is composed of aquatic animals that propel themselves through the water by beating cilia. (p. 648)
• Phylum Cycliophora is composed of tiny organisms that are so unusual they were assigned a new phylum. (p. 648)