2 A SYNOPSIS OF ANIMAL CLASSIFICATION. 



Others and represented a certain definite plan of structure having no 

 relation to the rest. Later investigation, however, has altered the 

 significance of the Types, and what were once considered as distinct 

 and unrelated animal groups are now seen to be but convenient divi- 

 sions, based upon the actual structure of living animal forms and of 

 the few extinct ones the remains of which have come down to us, 

 and more or less organically connected with one another. 



Among the modifications of this arrangement since the time of 

 Cuvier are (i) the introduction of several new types, and (2) the 

 grouping of the types thus established into larger groups in an 

 attempt to express the various interrelations which exist between 

 them. Thus the Protozoa, which are unicellular, and consequently 

 without tissues, are contrasted with the Metazoa, which are multi- 

 cellular, possess tissues and include all other animals. 



The Metazoa, again, are divided into those composed of but two 

 germ-layers and with a single internal cavity, and those composed of 

 three germ-layers and with one or more cavities (coelomes) interposed 

 between the central cavity and the exterior, the two groups thus repre- 

 senting a single and a double tube respectively. Of these the first in- 

 cludes the single Type of Ccelenterata (Type II), while the remainder 

 are taken together to form the contrasting group of Coelomata. 



These latter Types (III-VIII) are again divided into three subdivi- 

 sions, probably representing as many main branches or lines of descent : 

 (i) the Zygoneura, including Vermes, Mollusca, and Arliculata 

 (Types III-V), and characterized by a nervous system of paired 

 ganglia placed ventrally ; (2) the Ambulacralia, including the single 

 Type of Echinodermaia (Type VI), animals which are bilateral when 

 young and radiate when adult, and which are furnished by a water- 

 vascular system terminating externally in a series of ambulacral feet ; 

 and (3) the Chordata, which include the Preveriebrata and Verte- 

 brata (Types VII, VIII), and are characterized by the presence of a 

 dorsal nervous system and a central skeletal axis, the notochord, about 

 which in the higher forms an internal skeleton becomes developed. 



Certain of the Types include within their limits forms possessing 

 structural differences of too fundamental a character to allow of their 

 consideration as a single unified group, and yet conforming too closely 

 in general plan to allow the formation of new Types. In such cases it 



