THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 101 



ful observation of three classes of facts the correspondences 

 of parts in the successive ' somites " of an adult articulate 

 animal ; the still more marked correspondences of such 

 parts as they exist in the embryonic or larval articulate ani- 

 mal ; and the maintenance of such correspondences in some 

 types, which are absent in types otherwise near akin to them. 

 The nature of the conclusion which these evidences unite in 

 supporting, will best be shown by the annexed copies from 

 the lecture -diagrams of Prof. Huxley; exhibiting the 

 typical structures of a Myriapod, an Insect, a Spider, and a 

 Crustacean, with their relations to a common plan, as in- 

 terpreted by him. 



Head Thorax Abdomen 



/S7 



Treating of these homologies, Prof. Huxley says " that a 

 etriking uniformity of composition is to be found in the heads 

 of, at any rate, the more highly organized members of these 

 four classes, and that, typically, the head of a Crustacean, 

 an Arachnid, a Myriapod, or an Insect, is composed of six 

 somites (or segments corresponding with those of the body) 

 and their appendages, the latter being modified so as to 

 serve the purpose of sensory and manducatory organs." 

 And omitting the Myriapods, he also finds among these 

 groups the further unity that in most of them the entire 

 animal contains twenty of these homologous segments. 



