TRANSFORMATION OF ORGANS OF ANIMALS 31 



appendages of the different metameres (the somites, 



or constituent segments of the body) of the cray- 



fish are constructed on the same plan. The 



typical organ (fig. 2), when complete, consists of 



three parts the protopodite (pr.), . 



which is inserted into the body en _ J 



and carries a gill (br.) ; the endo- 



podite (en.) and the exopodite 



(ex.), each of which consists of a 



series of joints which are attached 



to the end of the protopodite 



the endopodite to the inner 



side, the exopodite to the outer. 



FIG. 2. Diagram of typical 



Let US nOW examine the and complete appendage 



of Astacus Jluviatilis : pr. 



twenty metameres of the animal protopodite; 6/-, giii ; 



ex, exopodite; en, endo- 



m succession. 



In each segment of the body we find a pair of 

 appendages, the structure of which is 

 based upon that of a complete ap- 

 pendage, but in which many different 

 adaptations have brought about the 

 conservation and increase of some of 

 the typical parts, and the partial or 

 FIG. 3. - Astacus total atrophy of other parts. 



Take first the abdominal segments, 



s e r g me n a t bd of mi t n h a etney carry appendages (fig. 3) formed 

 ?r ma p e rotopodite ; :of tne tnree fundamental parts: the 

 Spodfte protopodite (pr.), the exopodite (ex.), and 

 fo Q endopodite (en.). Each of these 

 parts is itself divided into different joints, the 



