THE NEWLY-HATCHED CRAYFISH. 219 



The posterior cephalic and the thoracic appendages 

 {5 — M) elongate and gradually approach the form which 

 they possess in the adult. I have not been able to 

 discover, at any period of development, an outer division 

 or exopodite in any of the five posterior thoi'acic limbs. 

 And this is a very remarkable circumstance, inasmuch 

 as such an exopodite exists in the closely allied lobster 

 in the larval state ; and, in many of the shrimp and 

 prawn-like allies of the crayfish, a complete or rudi- 

 mentary exopodite is found in these limbs, even in the 

 adult condition. 



When the crayfish is hatched (fig. 60) it differs from the 

 adult in many ways — not only is the cephalothorax more 

 convex and larger in proportion to the abdomen ; but the 

 rostrum is short and bent down between the eyes. The 

 sterna of the thorax are wider relativelj^ and hence there 

 is a greater interval between the bases of the legs than in 

 the adult. The proportion of the limbs to one another 

 and to the body are nearly the same as in the adult, but 

 the chelae of the forceps are more slender. The tips of 

 the chelas are all strongly incurved (fig. 8, B, p. 41), and the 

 dactylopodites of the two posterior thoracic limbs are hook- 

 like. The appendages of the first abdominal somite are un- 

 developed, and those of the last are inclosed within the 

 telson, which is, as has already been said, of a broad oval 

 form, usually notched in the middle of its hinder margin, 

 and devoid of any indication of transverse division. Its 

 margins are produced into a single series of short conical 



