CRUSTACEA. 191 



actual metamorphosis of the Crustacean in question, is most ac- 

 ceptable. He affirms a corresponding metamorphosis to occur in 

 the ditchprawn {Palemon variabilis) and common shrimp ( Crangon 

 vulgaris). Dr. Thompson has witnessed similar metamorphoses in 

 the genera Palinurus, Squilla, Pagurus, Porcellana^ Galatea, and the 

 marine species of Astacus, as well as in Palemon and Crangon. 



Finally, the metamorphosis of another species of shrimp (Cari- 

 dina 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 cor- 

 responds with that described by Rathke in the Astacus fluviatilis. 

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

 first steps in the definition of the alimentary canal and circulating 

 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 maxillas ; and the ambulatory thoracic legs precede 

 the masticatory pairs in their formation. The young Caridina, more- 

 over, is born with only three pairs of jaws, and the representatives 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 ^^^^ ; the sub- 

 sequent ecdyses are numerous, and take place at long intervals. It is 

 unquestionable that the Caridina, unlike the Craw-fish, is excluded 

 neither under the form, nor with all the parts which it possesses in its 

 mature shape. It wants, for example, the branchiae, a certain num- 

 ber of maxillae, the ambulatory thoracic, and the lamelliform ab- 

 dominal feet ; it possesses neither the squamous tail nor the complex 

 stomach of the mature creature. 



The cumulative evidence of the metamorphoses of Crustacea can 

 no longer be rejected : but their modifications in different genera, and 

 the number of the exceptions to the law, like that presented by the 

 Astacus fluviatilis ■, are yet to be determined. Here, therefore, is an 

 ample field open to the researches of the original observer, a field 

 which must be diligently and extensively cultivated before it can 

 yield the fruits of true generalisations as to the extent and nature 

 and varieties of the metamorphoses in the class of articulate animals 

 which support their bodies on jointed limbs and breathe by gills. 



