XIL] THE FRESH-WATER CRAYFISH. 139 



flattened sac lodged in the interior of the antennule. One 

 side of this sac is in-folded so as to produce a ridge, which 

 projects into the cavity of the sac, and is beset with very 

 fine and delicate hairs. The auditory nerve enters the fold, 

 and its ultimate filaments reach the bases of these hairs. 

 The sac contains water in which minute particles of sand 

 are suspended. 



The sexes are distinct in the Lobster and Crayfish. The 

 external characters of the males and females and the form 

 of the reproductive organs are described in the Laboratory 

 work. 



The impregnated ova are attached in great numbers, by 

 a viscid secretion of the oviduct, to the hairs of the swim- 

 merets, where they undergo their development. A Lobster 

 with eggs thus attached, is said by the fishermen to be 

 ' in berry.' In the Crayfish, the embryo passes through all 

 the stages which are needed to bring it very near to the 

 form of the adult before it leaves the egg: but, in the 

 Lobster, the young, when hatched, are larvae extremely un- 

 like the parent, which undergo a series of metamorphoses in 

 order to attain their adult condition. The larae may fre- 

 quently be obtained by opening the eggs of a ' hen-lobster ' 

 in 'berry.' They have a rounded carapace, two large eyes, 

 a jointed abdomen devoid of appendages ; and the thoracic 

 limbs are provided with long exopodites. 



The ordinary growth, no less than the metamorphoses 

 of the Lobster and Crayfish, are accompanied by periodical 

 castings of the outer, chitinous, layer of the integument. 

 After each such ecdysis, the body is soft and the animal 

 retires into shelter until the * shell ' is reproduced. 



