THE SHOllES OF BRITAIN. H^i 



The Crustaceous and Testaceous classes afford em- 

 ployment to a considerable number of our population, 

 and demand our brief attention. Of the former, the 

 chief species selected for food in this country are, the 

 Crab, the Lobster, the Prawn, and the Shrimp. Both 

 our salt and fresh waters, however, contain multitudes 

 of other species, some of which are exceedingly curious 

 in structure and form. The Crustacea, like insects, 

 have no internal skeleton ; but instead of it, are 

 encased in a jointed framework, resembling the plate 

 armour of our forefathers, of a texture between shell 

 and bone. The muscles which move the body are 

 attached to the interior of this crust, as our muscles 

 are attached to the bones. The body consists mainly 

 of two parts ; the fore-division contains the head and 

 chest, covered with a large single plate, and the 

 hinder, the belly covered with several smaller plates, 

 joined by a tough skin, and lapping over each other. 

 As this shelly covering is possessed by the animal 

 from its very birth, it is natural to inquire how it can 

 possibly increase in size, seeing it is enclosed in an 

 unyielding prison. In the Tortoises, which are some- 

 what similarly encased, the difficulty is met by a 

 periodical addition to the anterior surface of every 

 plate a little wider in diameter than the one before, 

 thus enlarging the capacity of the aggregated plates, 

 together with the enlargement of each plate ; and 

 this, as I have already observed, is the mode by which 

 the scales of a fish grow. But from the shaj^e and 

 size of the plates on a Crab or a Lobster, and espe- 

 cially of the great one that envelops the chest, this 



mode of growth would not answer the purposa 



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