NERVOUS SYSTEM OF VERTEBRATES. 



71 



The whole animal kingdom may be primarily divided 

 into seven groups called subkingdoms or departments or 

 phyla. These are again each subdivided into classes, and 

 these latter into orders, families, genera, species, etc. In 

 the schedule given below we go no further than classes. 

 Orders will be referred to sometimes, but not often. 

 Even some classes are not used. 



Metazoa. 



Proto- 

 zoa. 





Articulata. 



Mollusca. 



Radiata. 





Vertebrata. 



Arthropoda. 



Annelida. 



Echino- 

 dermata. 



Ccelen- 

 terata. 



Proto- 

 zoa. 



Mammals 



Birds 



Reptiles 



Amphibia 



Fishes 



Insects : 

 Arachnids 

 Myriapods 



Crustacea 



Annelids 



Cephalopods 

 Gasteropods 

 Acephala 

 Brachiopods 



Echinoids 

 Asteroids 

 Crinoids 

 Holothuri- 

 oids 



Acalephae 

 Polyps 



Infuso- 

 ria 



Rhizo- 

 pods 



These groups are not of equal value or significance, 

 as shown above. The whole animal kingdom may be 

 divided into two prime groups, viz., protozoa, or sim- 

 plest animals consisting of one cell only, and metazoa, or 

 animals consisting of an aggregate of more or less 

 differentiated cells. The metazoa, being higher, are 

 more differentiated, and therefore are divided into many 

 great departments. Again, I have linked together the 

 echinoderms and ccelenterates under the name radiata, 

 as having a common radiated plan of structure; also the 

 arthropods and annelids, or segmented worms, under 

 the name articulata, as having a common jointed or 

 ringed plan of structure. We shall use these terms in 

 connection with the general laws of animal structure. I 

 take for granted that the student already has some gen- 

 eral knowledge of zoology. I give only such classifica- 

 tions and such names as I shall use in the comparison 

 that follows. 



