Survey of the Animal Kingdom 293 



locomotor organs (Fig. 97) and their jellylike bodies. Other species are called 

 sea walnuts because of their walnutlike shapes. Number of species in the phylum 

 Ctenophora, 100. 



Classification of the Phylum Ctenophora 



Class 1 — Tentaculata (ten tak u -la' ta) (L. tentare, to feel). — These Cte- 

 nophores possess contractile, sensory tentacles. 



Class 2 — Nuda (nu' da) (L. nudus, bare or devoid of). — These Ctenophores 

 are without tentacles; the body is thimble-shaped; mouth and pharynx are large. 

 Example: Be roe (Fig. 97). 



Phylum 5. Platyhelminthes (plat i hel -min' thez) (Gr. platus, broad or flat; 

 helmins, intestinal worm) 



General Characteristics 



These animals are flattened dorsoventrally and possess bilateral symmetry. 

 The body is elongated and unsegmented (nonmetameric) . The animals are 

 triploblastic, having three primary germ layers, the ectoderm, mesoderm, and 

 entoderm. There is no true body cavity (coelom). Certain species have 

 branched, tubular intestines (gastrovascular cavity) with a mouth. The spaces 

 between the organs and the body wall are occupied by a connective tissue called 

 parenchyma. No anus is present. Certain species are parasites and pass through 

 a number of complex stages in the bodies of several species of animals during their 

 life cycle. Some flatworms live in fresh water; others, in salt water; a few are 

 terrestrial. Number of species of Platyhelminthes, 6,000. 



Classification of the Phylum Platyhelminthes 



Class 1 — Turbellaria (tur be -la' ri a) (L. turbo, disturb). — These turbel- 

 larians are free living in fresh, salt, or brackish water or moist soils. They possess 

 a ciliated ectoderm or epidermis. Special ectodermal cells produce rodlike bodies 

 known as rhabdites or they secrete mucus. They possess two prominent light-sen- 

 sitive eye spots. In general, they have remarkable powers of regeneration of lost 

 parts (Fig. 28) and also illustrate the phenomenon of axial gradient. 



Example: Planaria (Figs. 177, 178, and 179). 



Class 2 — Trematoda (tre ma -to' da) (Gr. trema, a pore; eidos, resemblance). 

 — ^The trematodes are flat and leaflike in shape and possess one or more ventral 

 suckers at or near the posterior end and in the anterior or mouth region. The 

 ectoderm is nonciliated but hardened in the adult. They are either endoparasites 

 or ectoparasites. 



Example: Liver fluke (Figs. 180, 181, and 374). 



Class 3 — Cestoda (ses -to' da) (Gr. kestos, a girdle; eidos, resemblance). — 

 The cestodes possess a scolex and a body made of a linear series of proglottids. 

 Each of these proglottids is really an individual in itself, so that the entire cestode, 

 strictly speaking, is unsegmented. All cestodes are endoparasites. They have no 

 mouth and no alimentary canal because of their parasitic habits. They inhabit 

 the alimentary canals of a great variety of vertebrate animals during some stage of 

 their life cycle. The cuticle of the adult is not ciliated. 

 Examples: Tapeworms (Figs. 182, 183, and 268). 



