$M ZOOGENESIS ^^ 



is commonly by division of the body into two equal parts, but 

 in some by the formation of buds which become detached, by the 

 breaking up of the body into "spores," or by other means; there 

 is, however, never any reproduction through the formation of 

 special sexual cells as in all other animal types. (Figs. 53, 

 p. 97; 87, p. 161.) 



The broader features of the interrelationships between the 

 major groups may be appreciated by a study of the following 

 key, bearing in mind that very many important structural char- 

 acters are not mentioned in the key. 



Key to the Major Groups of Animals 



a^ Body composed of a vast number of cells 

 b^ body with definite organs and structures 

 c^ symmetry completely, or almost completely bilateral; 

 body with a dorsal and ventral surface, and the two 

 halves on either side of a plane passing through the mid- 

 line alike 

 d^ no trace of radial symmetry 

 e^ with a vascular system 

 /^ with a body cavity; no protrusible proboscis the 

 sheath of which runs the whole length of the body 

 g^ no water vascular system 



h^ body never completely ensheathed in a tough 

 or hard segmented external skeleton; jaws 

 never formed of modified legs 

 i^ body not enclosed between a dorsal and ven- 

 tral shell; if enclosed between shells (as 

 in some mollusks) these are left and right 

 / with a notochord, a hollow dorsally 

 placed nervous system, and a pharynx 

 opening to the exterior by lateral pass- 

 ages 

 k^ notochord extending practically the full length of the body; 

 body segmented 

 l^ dorsal nerve cord extending for some distance in front of the 

 notochord, and expanded at its anterior end into a brain; 

 anterior portion of the axial skeleton forming a skull en- 

 closing the brain Vertebrata 



^67] 



