CRABS, PRAWNS, AND LOBSTERS 1377 



position can be indicated only by the presence of the 

 olfactory operculum. 



This important little organ varies in its construc- 

 tion in the different families of Crustacea. In the 

 crab it is a small movable appendage, situated at the 

 point of junction 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 protu- 

 berance, and it is not capable of being withdrawn 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 di- 

 rection of the opening is the same, inward and for- 

 ward, 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 



