354 SYNOPTIC COLLECTION. 



the left of the large plate over Section 13). The three 

 enlarged figures represent different stages in the life of 

 the young crab. In this larval condition the plump, 

 rounded cephalothorax is provided with a long dorsal 

 and frontal spine. The compound eyes are sessile, while 

 the appendages of both regions of the body are adapted 

 for swimming. The zoe'a develops into the so called 

 Megalops stage (PL 865, fig. 2 ; see also the middle fig- 

 ure of the plate over section 13) which resembles the 

 lobster in the general shape of the body. The append- 

 ages of the abdomen are still adapted for swimming, but 

 those of the cephalothorax have become transformed into 

 organs for walking and more efficient instruments for 

 catching food. The claws are developed, but at this 

 stage are equal in size. The large, compound eyes are 

 now on short, stout eye-stalks. 



After moulting again, this swimming animal becomes a 

 walking crustacean and is found at the bottom (see figure 

 on the right of the plate over Section 13). 



The adult crab (Nos. 866-868) is a fine illustration of 

 concentration of parts and organs. This is shown most 

 forcibly when this animal is compared with the lobster. 

 Here the body is short, broad, and flat. The abdomen is 

 reduced to a thin, small, often colorless, and altogether in- 

 significant part of the body, consisting of a limited number 

 of segments and usually hidden away under the cephalo- 

 thorax. Its appendages are likewise limited in number 

 and are used almost wholly for carrying the eggs. The ter- 

 minal pair of swimmerets has disappeared and the telson 

 is scarcely more than a vestige of its former self. The 

 reduction of the abdomen is suggestive of the cause that 

 has produced it ; viz.) the change of habit from a swim- 

 ming to a walking type of animal. The effects of this 

 habit are still further illustrated by the shape of the ceph- 

 alothorax and the structure of the well developed walking- 

 legs. 



All the appendages of the cephalothorax the four 



