172 CLASSIFICATION OF ANIMALS 



or diploblastic structure ; ectoderm and endoderm separated by a non- 

 cellular, supporting lamella or by a thick gelatinous mass; tentacles 

 about the mouth; stinging capsules, except in ctenophores; usually 

 an attached stage in the life-cycle. 



(e) Phylum Platyhelminthes. — Free-living representatives, the 

 planarians, and the parasitic flukeworms and tapeworms may have 

 been studied. Understand life-cycles, the degeneration of structures 

 related to free life, and the specialization of structures and functions 

 related to parasitism. Demonstration specimens of the ribbon-worms, or 

 nemerteans, should be examined. Distinguishing characteristics of the 

 phylum, as shown by its free-living members, are : greater cell speciali- 

 zation than in coelenterates ; bilateral symmetry; a single opening to 

 the digestive tract, except in the nemerteans; three germ layers, or 

 triploblastic structure, but no coelom; excretory structures in the form 

 of flame cells and ducts ; mostly hermaphroditic. 



(f) Phylum Nematoda. — Vinegar eels and other free-living round- 

 worms of this type, as well as parasitic representatives, such as Ascaris, 

 may have been studied. Trichina worms, hookworms, and filarial 

 worms are other examples. Understand life-cycles of the forms ex- 

 amined. The almost universal distribution and the economic impor- 

 tance of the parasitic species are of interest. Free-living species 

 abound in fertile soils (c/. Fig. 52, p. 106). Distinguishing character- 

 istics of the phylum are: elongated cylindrical body; bilateral sym- 

 metry; digestive tract with mouth and anus well developed; three 

 germ layers; body cavity; dorsal ganglion, nerve ring encircling the 

 pharynx, and ventral nerve cord ; sexes separate. 



(g) Phylum Nematomorpha. — Demonstration specimens of the 

 roundworm called the horsehair snake, Gordius, and its free-living 

 larval stage should be examined. Understand the life-cycle. The 

 distinguishing characteristics of the worms comprising this small group 

 are so like those of nematodes that the Nematomorpha are often 

 classified with the Nematoda in a single phylum. 



(h) Phylum Acanthocephala. — Demonstrations of the roundworms 

 called spiny-headed worms should be examined (c/. Fig. 52 D, 

 p. 106). Understand the life-cycles of representative species. The 

 distinguishing characteristics of the worms comprising this small group 

 are so like those of nematodes that the Acanthocephala are often 

 classified with the Nematoda in a single phylum. 



(i) Phylum Bryozoa. — Demonstrations of the marine bryozoan, 

 Bugula, and of fresh-water genera, such as Plumatella and Pectina- 

 tella, should be examined. Examine also the skeletons of encrusting 

 marine types. These "moss animals" consist of colonies composed of 



