100 



MORPHOLOGICAL DEVELOPIVIENT. 



their sub-kingdom, comparisons between the different ordera 

 prove that the higher are strongly distinguished from the 

 lower, by the much greater degree in which the individual- 

 ity of the tertiary aggregate dominates over the individual- 

 ities of those secondary aggregates called segments or 

 " somites,'^ of which it is composed. The successive Figs. 

 170 — 176, representing (without their limbs) a Julus, a 



I 3 



Scolopendra, an isopodous Crustacean, and four kinds of 

 decapodous Crustaceans, ending with a Crab, will convey at a 

 glance an idea of the way in which that greater size and 

 heterogeneity reached by the higher types, is accompanied 

 by an integration which, in the extreme cases, almost obliter- 

 ates all traces of composite structure. In the Crab the 

 posterior segments, usually folded underneath the shell, 

 alone preserve their primitive distinctness : so completely 

 confluent are the rest, that it seems absurd to say that a 

 Crab's carapace is composed of as many segments as there are 

 pairs of limbs, foot-jaws, and antennae attached to it ; and 

 were it not that during early stages of the Crab's develop- 

 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- 

 trme among naturalists. It is a doctrine that rests on care- 



