THE MORPHOLOGICAL COMPOSITION OP ANIMALS. H3 



ment the segmentation is faintly marked, the assertion might 

 be considered illegitimate. 



That all articulate animals are thus composed from end to 

 end of homologous segments, is, however, an accepted doc- 

 trine among naturalists. It is a doctrine that rests on careful 

 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 animal; 

 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 Crust- 

 acean, with their relations to a common plan, as interpreted 

 by him. 



Insecf 





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

 striking 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 



54 



