LESSON 36.] NUTRITION IN THE CRUSTACEA. 125 



In the pedunculated Cirripeds, a large granular, glandular mass 

 covers the viscera immediately beneath the muscular tunic of the 

 body ; its numerous ducts successively unite into three or four prin- 

 cipal trunks, which terminate in a lateral receptacle (h) at the side 

 of the intestine. In Lepas, a duct is continued from this receptacle, 

 which passes through the canal of the extensile tail (i). 



LESSON XXXVI. 



OKGANS OF NUTKITION IN THE CEUSTACEA. 



576. In these animals we find a class of articulata, in which the 

 segments of the body are provided with articulated (jointed) limbs, 

 or appendages. They are aquatic ; only a few of them being able 

 to support themselves, and that for a short time, on land. 



577. Their breathing organs, or branchiae, are organized for 

 aquatic respiration. 



578. In the highest individuals of the class, the Crab, and the 

 Lobster, the external layer of the integument is hardened by the 

 addition of earthy material, consisting of the carbonate, with a very 

 small proportion of the phosphate of lime. In the smaller and lower 

 Crustacea, the tegumentary covering retains a flexible, horny texture. 



In a great proportion of the class, the body consists of twenty- 

 one rings, of which seven are more or less soldered together to form 

 the head, seven more enter into the formation of the thorax, and 

 the remaining seven constitute the body. The great ponderosity of 

 the skeleton of the lobster, by the deposition of such inordinate 

 quantity of the Carbonate of lime, is essential to its well being, its 

 great weight having the effect of keeping the animal at the bottom, 

 notwithstanding the turbulence of the ocean. In these animals, the 

 smaller claw is provided with two rows of pointed, sharp, cutting 

 teeth, used as shears to divide its food ; the larger claw, on the con- 

 trary, is furnished with two or three short rounded tubercles, and 

 endowed as this claw always is with enormous muscular power, the 

 instrument becomes a most efficient anchor. 



579. The means of locomotion of a lobster in the water is pro- 

 digious ; it expands the five plates, constituting the terminal portion 

 of the body (or tail, as it is usually called), and by a single down 

 stroke it is propelled a distance of from 16 to 20 feet ! 



