CLASS CRUSTACEA. 27 



The Medicinal Leech is exclusively an inhabitant of fresh 

 water, and is mostly imported into Britain from Hungary, 

 Bohemia, Russia, and France. 



Recapitulation of Essential Chakactees. — The 

 body is elongated, and is ringed with transverse folds. 

 When lateral appendages are present, these are never dis- 

 tinctly jointed or articulated to the body. The nervous 

 system has the form of a chain of nervous masses placed 

 along the lower surface of the body, and united by longi- 

 tudinal cords. There is no distinct heart, but the circu- 

 latory system of the higher animals is represented, so far 

 as its function is concerned, by a peculiar system of 

 contractile tubes containing a coloured fluid. These char- 

 acters distinguish the class of the Annelida as a whole. 



CHAPTER IX. 



class CRUSTACEA. 



The class Crustacea comprises all the animals which are 

 commonly known as Crabs, Lobsters, Shrimps, Prawns, 

 Wood-lice, AVater-fleas, and the like, all of which have the 

 body enclosed in a more or less resistant shell. Hence 

 they are sometimes erroneously spoken of as " Shell-fish ; " 

 and hence also their scientific name (Latin, ci'usta, a crust 

 or shell). An admirable example of this class may be 

 found in the common Lobster (Homarus vidgaris) of 

 British seas. 



The Lobster is almost completely enclosed in a strong 

 and hard shell, which is really formed out of the skin, by 

 the deposition in it of lime and horny mattei'. A little 

 investigation also readily enables us to perceive that the 

 animal (fig. 1 2) is really composed of a number of rings 

 or distinct segments, placed one behind the other. Theo- 

 retically each segment is distinct, and each is constructed 

 upon a similar plan or model. In theor}'', namely, each 



