180 THE HARMONIES OF NATURE. 



provided with a trunk or cylindrical tube of variable length, 

 adapted for sucking, and in the interior of which are lodged the 

 mandibles, prolonged so much that they form two slender 

 and pointed processes, the extremities of which serve as a 

 lancet. Thus these fortunate parasites, who never know what 

 it is to want a meal, are enabled to tap the vessels of their 

 victims, and to quaff, without any further trouble, the rich 

 juices they afford ! 



But in the higher orders of crustaceans, whose food is gene- 

 rally of a solid and not easily digestible nature, the structure of 

 the oral apparatus is very different. The mouth is here furnished 

 with at least eight pieces or pairs of jaws, which pass the food 

 through an extremely short gullet into a membranous stomach 

 of considerable size. This stomach is rendered curious by having 

 within certain cartilaginous appendages, to which strong grinding- 

 teeth are attached. These are placed at the pyloric extremity 

 or outlet of the stomach, so that the aliment, after being sub- 

 jected to the action of the jaws, is again more perfectly chewed 

 by the stomach-teeth before entering the digestive tube, where 

 it is exposed to the action of th.e biliary fluid of the liver. The 

 different pieces composing the masticatory apparatus of the 

 stomach vary considerably in the different genera, and even in 

 the several species of the same genus; but in every case they are 

 always singularly in harmony with the kind of food taken, and 

 the general habits of the animal. 



The solid shell of the higher crustaceans completely encases 

 their body like a coat of mail. Unlike that of the sea-urchins, 

 which is formed of a multitude of small plates, constantly increas- 

 ing in diameter by the deposition of fresh calcareous matter 

 on their edges, and keeping pace with the growth of the animal, 

 it consists but of one piece, and is consequently incapable of 

 extension. Thus the lobster or the crab, after having once 

 attained their perfect form, would have been obliged ever after 

 to endure the confinement of a narrow garb, had not Providence 

 endowed them with the faculty of casting their shell from time 

 to time, and thus providing themselves with a new and more con- 

 venient tegument. A few days of fasting and sickness precede 

 the operation, during which the carapace becomes loosened 

 from the skin to which it adhered, and immediately begins to 

 secrete a new one soft and membranous at first, but soon be- 



