MUSEUM OF COMPARATIVE ZOOLOGY. 163 



The young remains in this embryonic condition for about twenty-four 

 hours (at least in confinement). In the mean while it has increased in 

 size to such a degree that the delicate investing membrane is no longer 

 ample enough for the enclosed zoea and the first exuviation takes place. 

 The cuticle of the abdomen is cast first, commonly coming off in one 

 piece (PL I. Fig. 9, 9'). The dorsal spine, which has been invaginated 

 like the parts already described, and laid forward over the back, begins 

 to be evaginated, and to erect itself, and thus aids in splitting the mem- 

 brane along the back. The rostrum, which has been applied to the 

 breast, also emerges, and the abdomen, freed from the embryonic cuti- 

 cle, is now used to clear the appendages of the cephalo-thorax, in this 

 wise : the ends of the two prongs of the tail-fork are bent so as to form 

 minute hooks (PL I. Fig. 12) : when the abdomen is flexed, these little 

 hooks catch in the membrane covering the cephalo-thoracic appendages, 

 and on extending the abdomen again the membrane is torn off (PL I. 

 Fig. 9). 



The dorsal horn is commonly evaginated, and assumes its position 

 with a slight backward curve even before the embryonic skin is entirely 

 got rid of. In specimens which have just cast the embryonic skin, a 

 break in the trend of the spine indicates the rim of the former invagi- 

 nation (PL I. Fig. 14). The rostral spine now projects downward at a 

 right angle with the long axis of the body. The seta? on the various 

 parts of the body unroll themselves, the mouth parts become functional 

 jaws, enabling the young animal to feed ; the two pairs of swimming- 

 feet, provided each with four long swimming-setae on their external 

 branches, become active agents for locomotion, and now, in place of the 

 inert and pupa-like embryo, we have a vigorous free-swimming larva. 



Besides the great difference between the two stages caused by the 

 sudden development of the dorsal and frontal spines, the two pairs of 

 antennae and the tail have an entirely different form. Both pairs of 

 antennae are now of relatively small size. Those of the first pair are 

 composed of but one segment, which carries three long sensory threads 

 at the tip. This segment corresponds to the basal segment of the first 

 antenna of the embryonic stage. 



The second pair of antenna? consist of a basal piece with a long ser- 

 rate spinous process (PL I. Fig. 10, a; PL II. Fig. 3, a), which lies in 

 the short, blunt process of the antenna of the embryonic stage (PL I. 

 Fig. 5, a), and a short, blunt protuberance (PL I. Fig. 10, c), the rudi- 

 ment of the antenna of the adult crab. 



In addition to these processes, there is articulated to the basal piece 



