NO. 10 CRUSTACEAN METAMORPHOSES — SNODGRASS 6l 



(1891) iti the metazoea of Piltimnus {fig. 22 C). The first and sec- 

 ond maxilhpeds still have the zoeal structure, but they are followed 

 by the third maxillipeds and five pairs of pereiopods, of which the 

 first are strongly chelate. Moreover, all these newly developed ap- 

 pendages except those of the last two pairs support branchial lobes 

 on their bases. Similar advanced larval instars are shown for several 

 other species of Brachyura by Lebour (1928). Hence, we should 

 assume that there must be in Callinectes a metazoeal instar in which 

 the thoracic appendages are in a state of development that could go 

 over at one moult into the appendages of the megalops. In the life 

 history of the crab there is no form corresponding to the mysis stage 

 of Penaeus (fig. 21 E) or that of Homarus (fig. 22 A), but the meta- 

 zoea might be likened to the postmysis of Penaeus. 



The megalops (fig. 23 E) is clearly a young crab, though it is only 

 a few millimeters in length. The dorsal spine of the zoea has been 

 shed with the larval cuticle (fig. 22 C) and the rostrum is shortened 

 to the ordinary length. The swimming maxillipeds are transformed 

 into feeding organs, and the other appendages are those of the adult. 

 The prominent stalked eyes give the megalops its name ("bigeye"). 

 An important feature of the megalops, however, is the extension of 

 the abdomen from the thorax, which suggests that the megalops 

 represents an adult ancestral form of the crab before the latter 

 permanently flexed its abdomen forward beneath the thorax. 



The adult crab on issuing from the cuticle of the megalops is still 

 a minute creature and goes through a large number of instars before 

 becoming sexually mature, after which it may continue to moult at 

 intervals. The habits of adult crabs are more various than those of 

 the larvae. While most adult crabs live in the ocean and crawl on the 

 bottom, some of them live in the shells of mollusks, in echinoderms, 

 in cavities of corals, and in tubes of worms. Others have left the 

 water for the land, where they dig deep burrows in the sand above high 

 water, and still others go freely inland, even invading human habita- 

 tions. The famous anomuran robber crab of the South Sea Islands 

 is said to climb cocoanut trees for their nuts. Regardless of their 

 habits or the nature of their dwellings, however, the brachyuran crabs 

 have undergone little structural adaptation. They vary in size and 

 shape, in the relative size of the chelae, and in the length of their 

 legs, but in general they retain the typical crab structure. Among the 

 Anomura, however, a pronounced adaptive modification of the body 

 occurs in the hermit crabs that live in snail shells. The carapace of 

 these crabs is weak and flexible. The abdomen is a long, soft sac that 



