HOMOLOGY AND HOMOLOGUES. 149 
aud, 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 crayfish 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- 
