NO. 10 



CRUSTACEAN METAMORPHOSES — SNODGRASS 



59 



zoea transforms into a preliminary crablike stage known as a megalops. 

 The life history of the blue crab of the Chesapeake Bay, Callinectes 

 sapidus Rathbun, has been studied by Churchill (1942), Hopkins 

 (1944), and Sandoz and Hopkins (1944), and is typical of the de- 

 velopment of most of the Brachyura. The young crab is sometimes 

 hatched in a final embryonic stage called by Churchill a pre zoea (fig. 

 23 A). It is still enclosed in a thin, transparent, closely fitting cuticle 



Fig. 22. — Decapoda: Macrura and Brachyura. Young stages. (A, B from S. I. 

 Smith, 1 871-1873; C from Cano, 1891.) 

 A, Homarus americanus H. Milne Edw., first larval instar, zoea. B, same, 

 third instar. C, Pilmnmis, a brachyuran crab, metazoea with partly developed 

 chelipeds and pereiopods. 



that covers the spines, which will be exposed at the first moult. Sandoz 

 and Hopkins say that emergence in the prezoeal stage results from 

 unfavorable conditions at the time of hatching. The first free larva 

 is a typical crab zoea (B) about 0.85 mm. in length. It has a short, 

 rounded carapace and a long, slender, segmented abdomen. The last 

 appendages are the large first and second maxillipeds, the exopodites 

 of which are equipped with terminal fans of long feathcrlike bristles. 

 The sixth segment of the abdomen is still united with the tclson. In 

 the second zoea (C) there is no essential change of structure, but the 



