II.] THE CRAYFISH AND LOBSTER. 237 



a. The appendages; paired differentiations of the 

 blastodermal area, separated by a median space 

 (more or fewer will be seen). 



^, The labrum; an oval median outgrowth, situated 

 about on a level with the first pair of appendages. 



y. The abdominal papilla ; now elongating. 



If the specimen be a young one the involution 

 to form the mouth may be seen immediately 

 behind the labrum; that to form the anus, under 

 cover of the abdominal papilla. If it be an 

 advanced one, segmentation of the papilla will be 

 very distinct. 



S. The optic pits; two small round differentiations 

 within the procephalic lobes {c. a.). 



e. The carapace; now appearing as a marginal thick- 

 ening of the blastoderm, immediately external to 

 the appendages. It is at first paired, the two 

 folds meeting behind the abdominal papilla. 



t. The hatched embryos. To be found in numbers, 

 fixed to the swimmerets of the mother. 



In general they resemble broadly the parent or- 

 ganism. Examine — 



a. The cephalo-thorax ; more arched than that of the 

 adult, owing to distension by the food-yolk, not 

 yet completely absorbed. The rostrum is bent 

 downwards and the cervical groove is, at best, ill 

 defined. 



[i. The cephalo-thoracic appendages ; differing but 

 little from those of the adult. Remove and 

 examine the free-end of the chela ; its pincers 



