EXERCISE ON CLASSIFICATION 133 



3 (2). Not sessile, nor stalked; moving on the oral surface. 4 



4 (7). Star-shaped, five to many rays. 5 



5 (6). Rays slender, sharply marked off from the disk. 



Class Ophiuroidea 



6 (5). Rays broad, not sharply separated from the disk. Class Asteroidea 



7 (4). Not star-shaped; spherical, oval, or flattened disks; very spiny. 



Class Echinoidea 



8 (i). Skeleton rudimentary; animals with leathery and tough body walls; 

 elongated, even wormlike, with a tendency to bilaterality; commonly with 

 branched tenacles around the mouth. Class Holothuroidea 



VI. Phylum Mollusca 



1 (2). With arms provided with cuplike suckers; shell apparently absent, 

 or, if present, spiral and divided into chambers. Class Cephalopoda 



2 (i). Without such sucker-bearing arms; shell never divided into chambers, 

 sometimes absent. 3 



3 (4, 5). Shell bivalve. Class Pelecypoda 



4 (3> 5)' Shell of eight pieces. Class Amphineura 



5 (3, 4). Shell univalve or absent. 6 



6 (7). Shell shaped like an elephant's tusk. Class Scaphopoda 



7 (6). Shell generally spirally coiled, sometimes cap-shaped or conical, 

 sometimes absent. Class Gasteropoda 



VII. Phylum Arthropoda 



1 (2, 5). With two pairs of antennae in front of the mouth; generally 

 covered with a hard exoskeleton; breathing by means of gills; usually with 

 numerous appendages; aquatic or in damp places. Class Crustacea 



2 (i, 5). With one pair of antennae; breathing by means of tracheae. 3 



3 (4). Elongated, wormlike, with one or two pairs of jointed walking legs 

 on each segment of the body. Class Myriopoda 



4 (3)- With only three pairs of jointed walking legs (larval forms of this 

 class are provided for in the key to phyla). Class Insecta 



5 (i, 2). Without antennae; four pairs of walking legs (in some cases 

 appearing as six pairs through leglike condition of mouth parts). 



Class Arachnida 



VIII. Subphylum Vertebrata 



1 (4). Fishlike animals, living in water and breathing by means of gills; 

 without lungs or limbs. 2 



2 (3). Without jaws, paired fins. Class Cyclostomata 



3 (2). With jaws and paired fins. Class Pisces 



