340 



LECTURE XV. 



which the larvae were produced in the months of March and April. 



The form under which they first appear is represented in^^. J38. : 



138 



% 



139 



Larva of Crab. Second stage. 

 Larva of Crab. First stage. 



they are then about half a line in length. Soon after exclusion this 

 larva casts off its envelope and assumes the appearance represented 

 in Jig. 139., which closely corresponds with that zoeaeiform Crustacean 

 whose further changes were witnessed by Thompson, and which he 

 had assured himself was an early or larval state of a common crab. 

 The last form which immediately precedes the assumption of the 

 mature characters corresponds, according to Thompson, with that of 

 the genus Megalopa. 



The additional evidence adduced, in 1839, by Capt.Du Cane in proof 

 of the actual metamorphosis of the Crustacean in question, was most 

 acceptable. He affirms a corresponding metamorphosis to occur in the 

 ditch-prawn {Palemon variabilis) and common shrimp {Crangon vul- 

 garis). Dr. Thompson has witnessed similar metamorphoses in the 

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

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



Mr. Couch, a medical gentleman residing on the coast of Cornwall, 

 contributed further conformation in a paper to the Cornish Natural 

 History Society in 1843*, in which he described, with much care and 

 detail, the metamorphoses of the common crab. The nervous system 

 first appears on the ventral aspect of the embryo ; at this period it 

 agrees in its arrangement with that which is persistent in the 

 edriophthalma, and the eyes of the larval crab are likewise now 

 sessile. A second metamorphosis takes place, when the eyes become 

 supported on long and thick peduncles. None of the phenomena are 

 more remarkable than that of the nervous system, where ten pairs of 

 ganglia are consolidated into one great central ganglion in the crab. 



* CCXXXIX. p. 28. 



