112 



MORPHOLOGICAL DEVELOPMENT. 





decided as to leave scarcely a trace of the articulate struc- 

 ture: the type being in these cases indicated chiefly by the 

 presence of those characteristically-formed limbs, which give 

 the alternative name Arthropoda to all the higher Annulosa. 

 Omitting the parasitic orders, which, as in other cases, are 

 aberrant members of their sub-kingdom, comparisons between 

 the different orders prove that the higher are strongly dis- 

 tinguished from the lower, by the much greater degree in 

 which the individuality of the tertiary aggregate dominates 

 over the individualities of those secondary aggregates called 

 segments or " somites," of which it is composed. The suc- 

 cessive Figs. 170 — 176, representing (without their limbs) a 



Julus, a 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 accom- 

 panied by an integration which, in the extreme cases, nearly 

 obliterates 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 antennas attached to it; and 

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





