CHAPTEE VIII. 

 THE DECAPOD CRUSTACEA. 



General characters of Crustacea Structure of prawn, lobster, and 

 crab Classification of Decapod Crustacea Swimming and creeping 

 forms Common British shrimps and prawns. 



IJS" this chapter we have to consider one of the most inter- 

 esting classes in the animal kingdom, interesting alike on 

 account of the beauty of form and colour, of the structure 

 and the habits. The class Crustacea is a very large one, 

 and embraces a great variety of animals adapted for many 

 different habitats and modes of life. Like the insects on 

 land, the Crustacea seem to display every possible modifica- 

 tion of parts; if they are less popular than insects it is 

 certainly not because they display fewer points of interest 

 or less beauty. 



They resemble insects in being clothed in an envelope of 

 chitin, which invests the whole body, and is inturned to 

 line part of the alimentary canal and to form the tendons of 

 the muscles. This chitinous coat gives great definiteness 

 of form the Crustacea never exhibit the variability of 

 shape which often makes the study of soft-skinned animals 

 so difficult; it has also such an intimate connection with 

 the internal organs that the external appearance may be 

 used as a test of affinity. In this respect the Crustacea, 

 or indeed the Arthropoda in general, differ markedly from 

 Molluscs. The shell of the latter has no very intimate 

 connection with the internal organs, it in itself yields little 

 information as to the anatomy of the contained animal. 

 In consequence, the structure and affinities of Molluscs can 

 be made out by dissection alone, and dissection, moreover, 

 which is often tedious and difficult even for trained fingers. 



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