174 EVENINGS AT THE MICROSCOPE. 



is raised, the internal surface is found to be perforated by 

 a circular opening protected by a thin membrane. 



In the Prawn, Shrimp, and Lobster, there is no oper- 

 culum, but only the orifice covered by a membrane, which 

 is placed at the extremity of a small protuberance, and is 

 not capable of being withdrawn into the cavity of the an- 

 tenna, as in the Crab. 



In the latter animal, the little door, when it is raised, 

 exposes the orifice in a direction pointing to the mouth ; 

 and where there is no door, still the direction of the open- 

 ing is the same, inwards and forwards, answering to the 

 position of the nostrils in the higher animals. In each 

 case it is so situated that it is impossible for any food to 

 be conveyed into the mouth without passing under this 

 organ ; and there most conveniently the animal is enabled 

 to judge of the suitability of any substance for food, by 

 raising the little door and applying to the matter to be 

 tested the sensitive membrane of the internal orifice. 



Thus it is concluded that this lower or outer pair of an- 

 tennae are the proper organs of smell, as the upper and 

 inner are of hearing.* 



The eyes, though constructed on the same general prin- 

 ciples as those of Insects, yet present some particulars 

 worthy of your notice. In the Crabs and Lobsters they 

 consist of numerous facets, behind each of which is a 

 conical or prismatic lens, the round extremity of which is 

 fitted into -a transparent conical pit, corresponding to a 

 vitreous body, while the conical extremity of these lenses 

 is received into a kind of cup, formed by the filaments of 

 the optic nerve. Each of these filaments, together with 

 its cup, is surrounded by pigment matter, in a sheath -like 

 manner. To see this structure would require anatomical 

 skill ; but you may here examine with a low power por- 

 tions of the cornea, or glassy exterior, of the eye of 

 a Crab, and of a Lobster. In the former, you see that 



* Op. cit. 



