TRANSFORMATION OF ORGANS OF ANIMALS 35 



second metamere, where degeneration was exhibited 

 by the relatively small size of the exopodite. 



It may be concluded from this examination that 

 in the abdominal appendages there is a degenera- 

 tion common to all this is the absence of the 

 poclobranch. In accordance with the particular 

 adaptations of each pair of appendages, special 

 degeneration accompanies special adaptive develop- 

 ments as we have shown above. 



We now pass to the fourteen anterior metameres 

 of the cray-fish. The first six of these constitute 

 the head, and carry on the first segment the stalked 

 eyes ; on the second, the antennules ; on the third, 

 the antennae ; on the fourth, the mandibles ; on the 

 fifth and sixth, the two pairs of maxillae. The 

 eight metameres of the middle of the body form 

 the thorax, and carry on the seventh, eighth, and 

 ninth, the maxillipedes ; on the tenth, the claws ; 

 and on the eleventh, twelfth, thirteenth, and four- 

 teenth, the walking legs. 



The structure of all these appendages may be 

 referred to that of the typical primitive appendage. 

 First take the thoracic appendages, commencing 

 with the third maxillipede (fig. 8) which is the 

 most complete appendage. It consists of a proto- 

 podite formed of two parts (coxopodite and basipodite) 

 and carries a podobranch (br.), a well-developed 

 endopodite (en.) consisting of five jointed parts, and 

 of a small exopodite segmented like those of the 

 abdominal appendages. 



C 2 



