CRUSTACEA. 341 



With respect to the 3Iaia, or spider-crab, also, there is a very com- 

 plete metamorphosis. According to Couch, in both kinds of crab the 

 larva quits the ovum in a form more strictly aquatic than the adult ; 

 and this gives us an insight into the curious law of the periodical 

 migrations of the land-crabs ( Gecarcinus) of the West Indies to the 

 sea. 



Brown, in his " History of Jamaica," gives a graphic account of 

 these migrations About the month of February or March they 

 may be seen in crowds on their way to the sea-side, impelled thereto 

 in order to deposit their ova in the sea. These ova have been ex- 

 cluded from the oviduct, and are attached, by the nidamental gluten, 

 to the ciliated sub-caudal appendages ; and Brown's idea was, that it 

 required the sea to wash them off. But now, as we know that the 

 larval land-crabs are natatory and more especially fitted for sea life, 

 we see the necessity for the migration of the parent-crab to that 

 element for oviposition. 



Finally, the metamorphoses of another species of shrimp ( Caridina 

 Desmarestii) have been described with all the requisite care and 

 detail by M. Joly, in the " Annales des Sciences Naturelles" for 

 January, 1843. The development of the ovum, up to the period of 

 exclusion and attachment to the maternal ciliated plates, closely 

 corresponds with that described by Rathke in the Astacus Jluviatilis. 

 The first stages in the formation of the rudimental extremities, — 

 the first steps in the definition of the alimentary canal and circu- 

 lating system, were likewise the same ; the heart was observed to 

 beat thirty-five times in a minute in the embryo Caridina. But the 

 formation of the abdomen is anterior to that of the antennae, the 

 labrum, and the maxillae ; and the ambulatory thoracic legs precede 

 the masticatory pairs in their formation. The young Caridina, 

 moreover, is born with only three pairs of jaws, and the represen- 

 tatives of the ambulatory feet are bifid, like those of the Mysis, and 

 are at first likewise only in three pairs. The abdominal segments 

 are without any vestige of lamelliform limbs. 



The bifid feet of the larva are metamorphosed into auxiliary jaws, 

 and the later bifid thoracic limbs are metamorphosed into the ordi- 

 nary ambulatory legs. With respect to the branchiae, they are not 

 at all developed when the young Caridina quits the ovum. The 

 first moult takes place three days after exclusion from the Qgg ; the 

 subsequent ecdyses are numerous, and take place at long intervals. 

 It is unquestionable that the Caridina, unlike the Craw-fish, is ex- 

 cluded neither under the form, nor with all the parts which it pos- 

 sesses in its mature shape. It wants, for example, the branchiEB, a 

 certain number of maxillae, the ambulatory thoracic, and the lamelli- 



z 3 



