CLASSIFICATION OF ANIMALS 19 



tubular digestive system, with a mouth at one end and an 

 anus at the other. Around this tube there is a fluid-filled 

 space, the ccelom or body-cavity, -which intervenes between 

 it and the solid body wall. The earthworm is again a good 

 illustration of this point. 



6. Appendages. Many animals possess appendages to 

 the body proper in the form of paddle-like swimming 

 organs, fins, jointed legs, etc. 



7. The Type of Skeleton (Fig. 17). Animals may have 

 hard structures on the outside (exoskeleton) or inside 

 (endoskeletori) of the body. In the vertebrates and their 

 relatives the skeleton is typically of the latter type and 

 its central axis always originates as a rod-like organ, the 

 notochordj or chorda, which extends longitudinally through 

 the body. In the vertebrates proper the chorda is re- 

 placed during early development by bony vertebrae which 

 form the spinal column, or " backbone." 



Using such criteria, it is possible to divide the animal 

 kingdom into fifteen great groups, or phyla, which may be 

 identified by means of the following "key": 



KEY TO PHYLA OF ANIMALS* 



1 (2) Body composed of one cell or a colony of similar cells; mostly 



microscopic Phylum I, PROTOZOA. 



2 (1) Body composed of many cells arranged in tissues 3 



3 (24) Body non-metameric 4 



4 (11) Radially symmetrical. Have proximal (attached) and distal 



(free) ends, or have oral (mouth) and aboral surfaces, but have 

 no head or tail. Parts of body radiate around a chief axis. . .5 



5 (6) Body filled with many minute pores; no definite mouth. 



Sponges Phylum II, PORIFERA. 



6 (5) Body not filled with pores ; a mouth present 7 



7 (8) Body with eight rows of comb-like plates arranged radially. 



Phylum IV, CTENOPHORA. 



8 (7) Body without eight rows of radially arranged paddle-plates. . 9 



9 (10) Body sac-like, often with tentacles. Polyps, medusae, jelly- 



fishes, corals, sea-pens. .. .Phylum III, CXELENTERATA. 



* In using the key there are always two alternatives. The first number 

 is to be compared elsewhere with the one in parenthesis after it. 



