202 EVENINGS AT THE MICROSCOPE 



the different families of Crustacea. In the Crab it is a 

 small movable appendage, situated at the point of junc- 

 tion between the second and third joints ; it is attached to 

 a long calcareous lever-like tendon, at the extreme limit 

 of which is placed a set of muscles, by which it is opened 

 and closed; to assist in which operation, at the angle of 

 the operculum most distant from the central line of the 

 animal are fixed two small hinges. When the operculum 

 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 

 it is not capable of being withdrawn into the cavity of 

 the antenna, 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, inward and forward, answering to the po- 

 sition 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 sen- 

 sitive membrane of the internal orifice. 



Thus it is concluded that this lower or outer pair of 

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

 inner are of hearing. 1 



The eyes, though constructed on the same general prin- 



1 Op. cit. 



