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 and is not capable of being withdrawn into the cavity of 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 antennae, 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 

 antenna? are the proper organs of smell, as the upper 

 and inner are of hearing.* 



The eyes, though constructed on the same general 

 principles 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 portions of the cornea, or glassy 

 exterior, of the eye of a Crab, and of a Lobster. In the 



* Op. cit. 



