350 SYNOPTIC COLLECTION. 



second, third, fourth, and fifth segments, 1 and the exter- 

 nal skeleton of the body is quite firm. 



In the stage preceding the moult when the animal seeks 

 a shell, the distinctly segmented abdomen has lost its 

 symmetry ; the feet are largest on the right side and the 

 abdomen begins to curve away from the longitudinal axis. 

 With the next moult, the abdomen is soft and the seg- 

 ments indistinct, while some of the abdominal appendages 

 are lost (PI. 856, Eupagurus pollicaris}. 



The adult usually lives in a Gastropod shell (No. 854; 

 No. 857, Petrochirus granulatus Stimp., see lower shelf) 

 and in this way the abdomen has become extremely soft, 

 light-colored, and indistinctly segmented. Its appendages 

 have lost all those characters which are natatory, arid have 

 either disappeared or have taken on secondary structural 

 features. Thus the first segment has no appendages ; 

 the second, third, and fourth have limbs on the left side 

 only for holding the eggs during the egg-bearing season ; 

 the fifth appendage is a mere vestige, while the sixth pair 

 are modified for holding the crab in its shell. 



The cephalothoracic appendages, though less modified 

 than the abdominal, still have undergone changes in 

 structure. The arms are used for walking, for getting 

 food, and for closing the aperture of the shell ; the second 

 and third pairs of legs for walking only, while the fifth 

 and probably the fourth pair aid in holding the animal in 

 its shell. 



Many interesting modifications of structure may be seen 

 in the burrowing crustacean, Hippa talpoida (No. 858 ; 

 PL 859). The smooth, more or less tubular cephalothorax 

 is well fitted for a burrowing animal. Four of the abdo- 

 minal segments can be seen from above, while most of 

 the remaining portion that is bent under the cephalo- 

 thorax is apparently composed of one piece, the segments 



1 Verrill, Rep. U. S. Comm. Fish and Fisheries, I, i87i-'72, p. 

 530- 



