EXERCISE ON CLASSIFICATION 129 



numbers which refer to the alternative statement. If, therefore, upon reading 

 the first statement the student decides that this does not fit the animal in ques- 

 tion, he turns at once to the number or numbers in parentheses and continues 

 to do this until he finds a statement that does fit. In this case he proceeds to 

 the number given at the end of the statement. This method of making a key 

 is taken from Ward and Whipple's Fresh-Water Biology. The key is in part 

 derived from one devised by Dr. V. E. Shelford, of the University of Illinois. 

 Only the simplest possible characters have been chosen as means of identifica- 

 tion, even though this often involves repetition and increased length of the key. 



Key to the Phyla of Animals 



1 (2). Animals consisting of a single cell, or of a colony of like cells, or masses 

 of multinucleate protoplasm; mostly microscopic. Phylum Protozoa 



2 (i). Animals consisting of many cells, of several or many different kinds, 

 arranged in definite layers. 3 



3 (4, n). Without definite symmetry. Forming sessile motionless crusts 

 or masses, often branching; body porous, rough, and bristly, pierced by numer- 

 ous holes, of which one or more are large and conspicuous. Phylum Porifera 



4 (3, n). With definite radial symmetry. 5 



5 (10). Relatively simple animals, without anus, coelome, or definite 

 organs. 6 



6 (7). Sessile, vase-shaped, or cylindrical forms, porous, rough, bristly, with 

 one large terminal, non-closable opening; without tentacles. A few members 

 of the Phylum Porifera 



7 (6). Soft, often gelatinous animals, not porous or bristly; apical opening 

 a closable mouth; nearly always with tentacles. Parts of the body arranged 

 in fours or sixes or indefinite. 8 



8 (9). With eight radial rows of ciliated swimming plates; tentacles, if 

 present, without nematocysts. Phylum Ctenophora 



9 (8). Without such rows of ciliated plates; with tentacles armed with 

 nematocysts; often sessile and colonial, some free-swimming. 



Phylum Coelenterata 



10 (5). More complex animals, with anus, coelome, and definite organs; 

 parts of the body almost always in fives, sometimes indefinite; hard, spiny, or 

 leathery animals; tentacles, if present, branched, and never with nematocysts. 



Phylum Echinodermata 



11 (4, 3). With definite bilateral symmetry, at least in part of the body; 

 sometimes spirally coiled in part; sometimes posterior end bent anteriorly 

 toward mouth. 12 



12 (34, 37). Without an internal cartilaginous or bony skeleton in the form 

 of skull or vertebral column, wall of the pharynx not pierced with gill slits. 13 



