XII.] THE FRESH- WATER CRAYFISH. 133 



elements of the retina of the Vertebrata which make up 

 the layers of rods and cones and the granular layers. These 

 structures are properly modifications of the epidermis; 

 inasmuch as the cerebral vesicle, of which the retinal vesicles 

 are outgrowths, are involutions of the epidermis of the 

 embryo, and, morphologically speaking, the free ends of the 

 rods and cones of the vertebrate eye are, as in the crustacean, 

 turned outwards. It seems probable, therefore, that the 

 crustacean eye is to be compared to the retina alone of 

 the vertebrate eye, and that vision is performed as it would 

 be by the retina deprived of its refractive adjuncts. 



The auditory organ of the Lobster and Crayfish is 

 situated in the basal joint of the antennule, on the dorsal 

 surface of which a small slit-like opening, protected by 

 numerous hairs, is to be seen. The chitinous layer of the 

 integument is invaginated at this opening, and thus gives 

 rise to a small flattened sac lodged in the interior of the 

 antennule. One side of this sac is in-folded so as to pro- 

 duce 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, 



