236 GEOLOGICAL BIOLOGY. 



terata, Echinodermata, Molluscoidea, and Mollusca are all 

 associated together by the fact that their development of 

 separate parts is in a direction radiately, or in circle about the 

 mouth, and hence they are antimeric Metazoa. 



The Metameric and Diarthromeric Types. — The Vermes and 

 the Arthropoda are, on the contrary, mctanicric ; the de- 

 velopment adds parts by repetition longitudinally along the 

 median axis. In the Vertebrates, in which there is added, 

 as Von Baer already saw, the diartJironicric separation of a 

 dorsal and ventral cavity, with specialized parts distributed in 

 each, the arrangement of specialized organs is on a monomeric 

 plan, as in some of the Mollusca. 



Meaning of Typical Structure and Types in Modern Zoology. 

 — It may be remarked that the difference between the old 

 and the more modern use of such classifications, as above 

 made, consists in the theoretical value placed upon them. 

 Cuvier and Agassiz considered such " types" as in the nature 

 of "ideal plans" which all animals for some reason were 

 obliged to conform to, and departure from the "type plan" 

 or " arche type" was an abnormality, or required tneoretical 

 adjustment to the plan. 



In modern Zoology by the " typical " structure of a group 

 is meant a generalized statement of the most conspicuous 

 features observed in the members of the class under con- 

 sideration, and departure from the type in individual cases is 

 evidence not of aberration in the particular case, but of imper- 

 fection of the description. The fact that there is develop- 

 ment along one or another line is important : the generaliza- 

 tion of the law so as to cover the principle and omit the 

 details aids the formation of clear notions; but the notions 

 are not the things, and the latter have to be constantly recti- 

 fied to express the increase of knowledge of the former. The 

 four types of Cuvier had their representatives in nature, but 

 all organisms did not stick closely to a particular type of con- 

 struction expressed by his formulation of characters of the 

 types. 



