174 CLASSIFICATION OF ANIMALS 



characteristics are: digestive tract with mouth and anus; three germ 

 layers; body cavity ill-defined; excretory tubules with flame cells; 

 no circulatory or respiratory systems; nervous system simple; the 

 ciliated corona, in most species; the mastax; minute size; sexes sepa- 

 rate; two kinds of eggs. 



(1) Phylum Annelida. — The earthworm and fresh- water genera, 

 such as Tubifex, are representative, although the more typical anne- 

 lids are forms with lateral appendages on most of the somites and 

 conspicuous sense organs in the head region. In these respects the 

 clamworm, Nereis, is a better example. Examine charts or textbook 

 figures of marine, tube-building annelids, such as Serpula. The 

 leeches are greatly modified annelids; the gephyrean worms, such as 

 Phascolosoma, are so different from other annelids that they are often 

 classified as a small phylum. Examine specimens of leeches and 

 gephyreans and understand their structure with aid of textbook ac- 

 counts. Distinguishing characteristics of the phylum are: bilateral 

 symmetry; obvious metamerism; three germ layers; an extensive 

 ccelom; paired nephridia in most of the somites; closed circulatory 

 system; dorsal brain, circumpharyngeal ring, and ventral nerve cord; 

 well-organized tissues and organ-systems, but very little head develop- 

 ment or cephalization. The gephyreans present notable exceptions to 

 these characteristics. 



(m) Phylum Mollusca. — Mussels, snails, limpets, chitons, squids, 

 devil-fishes, and cuttle-fishes may be examined. Understand from 

 textbook accounts, or otherwise, the mode of life in each .species thus 

 studied; and note the modification of the molluscan type of structure 

 that occurs in each class of the phylum. Distinguishing characteristics 

 are: bilateral symmetry; no metamerism; three germ layers; a ccelom 

 and nephridia; open circulatory system, except in cephalopods; paired 

 ganglia located in regions of greater bodily activity and joined by 

 paired commissures and connectives; forms, such as the cephalopods, 

 with a high degree of cephalization; a dorsal shell; a ventral foot; 

 a mantle enclosing a cavity into which gills extend. 



(n) Phylum Echinodermata. — Examine starfishes and sea-urchins, 

 noting the radial symmetry. Compare these animals with a sea- 

 cucumber and with figures of crinoids, noting the difference in rela- 

 tion of body axes to substratum. How would you homologize the 

 outer surfaces of a sea-urchin, a sea-cucumber, and a starfish? The 

 echinodcrms are an aberrant group whose radial symmetry is consid- 

 ered to be superimposed upon a bilateral symmetry. Examine speci- 

 mens or charts showing bilaterally symmetrical larval stages. Dis- 

 tinguishing characteristics of the phylum arc: radial symmetry, mask- 



