456 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



except the first, are two well-developed pleopods or swimming appen- 

 dages ; each of these consists of basal segment, exopodite, and endop- 

 odite. (Fig. 27.) The exopodites carry from 7 to 14 plumose swim- 

 ming hairs. The endopodites (fig. 28) are small and bear two or 

 three curly processes which are said to serve to lock together the 

 pleopods of a pair and insure their beating synchronously. When 

 the abdomen is extended and all the pleopods are beating together 

 they serve as a very effective locomotive apparatus, and the megalops 

 is driven forward with great swiftness. It is perhaps the swiftest 

 swimmer of all the minute animals at the surface. Not many of the 

 crab larvae lose their lives during the megalops stage. 



The megalops swims about for nearly a month. Unlike the zoea it 

 does not go through a series of molts during this time, although it 

 undergoes considerable internal development. After some three or 

 four weeks of its roving existence these internal changes begin to 

 affect its swimming powers. The swimmerets or pleopods begin to 

 shrivel up slightly. After this begins the megalops is glad to find 

 some convenient place to cling and hide. Such places are most com- 

 monly found in the shallow water near the shore where a rotting 

 board already honeycombed by the tiny tunnels of the wood-boring 

 amphipod (Chehira) or an encrusted oyster shell may offer an ideal 

 hiding place. This loss of the power of the pleopods for swimming 

 marks the end of the sea life and adventures of the larva. When these 

 organs lose their function it will never again be able to swim. 



The megalops may spend several days or even a week crawling 

 around in its new and temporary shelter. The flesh of its pleopods 

 is withdrawing more and more from the hairy chitinous sheath that 

 made them organs of swimming. At length the megalops molts and 

 out of the megalops shell crawls the first tiny fiddler crab although 

 even at this stage it could hardly be recognized as a fiddler although 

 it is undoubtedly a crab. 



CHANGE TO THE YOUNG CRAB. 



The young crab (fig. 32) differs most from the megalops in the 

 shape and relative size of its thoracic region and in the changed 

 abdomen. The thoracic region is now distinctly flattened and nearly 

 as broad as it is long. It is quite different in detail, however, from 

 that of the adult fiddler. Its lateral margin forms a wavy line instead 

 of being straight. Up to this time the developing larva has retained 

 practically the same distribution of pigment spots as was character- 

 istic of the earliest zoea. The spots are larger in the megalops and a 

 few additional spots appear in the thoracic region but even in the 

 megalops the distribution of the spots coincides very closely with that 

 of the first zoea. In the earliest crab, however, the entire body is 

 closely covered by numerous small pigment spots rather uniformly 



