EXERCISE ON CLASSIFICATION 131 



30 (20). Not tapelike; with anus, mouth, and digestive tract; segments 

 decreasing in size posteriorly or of the same size throughout. 31 



31 (32). Segments numerous, usually decreasing in size posteriorly; without 

 tracheae, tracheal gills, or spiracles ; never with definitely differentiated chitinized 

 heads; often with segmental bristles, but these never in large tufts at one end 

 of the body. Phylum Annelida 



32 (31). Segments few in number, not exceeding eleven or twelve; not 

 markedly decreasing in size posteriorly; with tracheal tubes, tracheal gills, or 

 spiracles; often separate hard chitinized heads; and may have bristles in tufts 

 at one end. Phylum Arthropoda (insect larvae) 



33 (28). With jointed appendages, at least on the anterior segments of the 

 body. Phylum Arthropoda 



34 (12, 37). Wall of the pharynx pierced with gill slits; without an internal 

 cartilaginous or bony skeleton. 35 



35 (36). Small, fishlike forms; sides of the body marked with zigzag muscle 

 segments; mouth without jaws, in the center of a rounded hood. 



Phylum Chordata; Subphylum Cephalochorda 



36 (35). Inclosed in a saclike gelatinous or tough coat, which has two 

 openings to the outside, for the ingress and egress of water; sessile, solitary or 

 colonial (the latter quite small), or free-floating, marine. 



Phylum Chordata; Subphylum Tunicata 

 37. With a cartilaginous or bony skull and vertebral column. 



Phylum Chordata; Subphylum Vertebrata 



Key to the Classes of the Principal Phyla 

 I. Phylum Protozoa 



1 (9). With temporary or permanent extensions of the surface of the body, 

 mostly for locomotor purposes. 2 



2 (3, 6). Extensions in the form of changing pseudopodia, blunt to long 

 and threadlike. Class Sarcodina (Rhizopoda) 



3 (2, 6). Extensions as raylike, non-motile (or nearly so) projections. 4 



4 (5). Rays with a terminal knob. Class Suctoria 



5 (4). Rays pointed, without a knob. Class Sarcodina 



6 (2, 3). Extensions in the form of long or short, hairlike, very active pro- 

 cesses. 7 



7 (8). Hairlike processes short and numerous (cilia). Class Ciliata 



8 (7). Hairlike processes long and few (one or two to several). 



Class Flagellata 



9 (i). Without locomotor or other processes of the body in the adult state; 

 parasitic. Class Sporozoa 



