HOMOLOGY AND HOMOLOGUES. 149 



and, in those of the male, as if all the rooms were run 

 into one. 



It is further to be remarked, that, just as of a row of 

 houses built upon the same plan, one may be arranged so as 

 to serve as a dwelling-house, another as a warehouse, and 

 another as a lecture hall, so the homologous appendages 

 of the cra3'fish are made to subserve various functions. 

 And as the fitness of the dwelling-house, the warehouse, 

 and the lecture-hall for their several purposes would not 

 in the least help us to understand why they should all be 

 built upon the same general plan ; so, the adaptation of 

 the appendages of the abdomen of the crayfish to the dis- 

 charge of their several functions does not explain why 

 those parts are homologous. On the contrary, it would 

 seem simpler that each part should have been constructed 

 in such a manner as to perform its allotted function in 

 the best possible manner, without reference to the rest. 

 The proceedings of an architect, who insisted on con- 

 structing every building in a town on the plan of a 

 Gothic cathedral, would not be explicable by considera- 

 tions of fitness or convenience. 



In the cephalothorax, the division into somites is not 

 at first obvious, for, as we have seen, the dorsal or tergal 

 surface is covered over by a continuous shield, distin- 

 guished into thoracic and cephalic regions only by the 

 cervical groove. Even here, however, when a transverse 



section of the thorax is compared with that of the abdo- 

 8 



