200 ETEITINGS AT THE inCKOSCOPE. 



joints of the stem are distinct ; but in the Crah the 

 whole are, as it were, soldered together into a compact 

 mass, with difficulty distinguishable into their constit- 

 uent articulations ; while in some species their 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 mem- 

 brane. 



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

 operculum, but only the orifice covered by a mem- 

 brane, which is placed at the extremity of a small pro- 

 tuberance, and it is not capable of being withdrawn 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 opening is the same, inwards and forwards, an- 

 swering to the position of the nostrils in the higher 

 animals. In each case it is so situated that it is im- 

 possible for any food to be conveyed into the mouth 

 without passing under this organ ; and there most con- 

 veniently the animal is enabled to judge of the suit- 



