INTRODUCTION. xi 



The construction of the skeleton in this class of animals 

 is for the most part very distinct from that of all others, 

 although in some of the abnormal forms there is a remark- 

 able deviation from the typical structure, and a corre- 

 sponding- approximation to that of other classes ; as, for 

 instance, in the segments of certain Isopoda, which re- 

 semble, in general character, some forms amongst the 

 Myriapoda. In the greater number of them, and es- 

 pecially in the higher forms, the tegumentary skeleton is 

 formed of a hard, solid, calcareous crust, the earthy por- 

 tion of which consists of carbonate of lime, with a small 

 portion of phosphate of the same earth. The colours by 

 which the crust is, in many cases, very beautifully marked, 

 depend upon a pigment which pervades different parts of 

 the substance, and offers various hues, and sometimes 

 curious and grotesque markings, in different species. The 

 colouring matter, in these as in most other animals, is 

 more intense on the upper than on the under surface, 

 the latter being, in many, nearly pure white, whilst 

 the former is deeply and brightly coloured. The earthy 

 matter is deposited upon, and produced by, an organized 

 vascular membrane or corium. In many of the smaller 

 Crustacea, even amongst the higher forms, as in most of 

 the PalfEinonidee or prawns, and other allied families, as 

 well as in most of the lower groups, as the Isopoda, and 

 others, the crust retains its semi-transparent, elastic, and 

 flexible nature, resembling thin horn or parchment, 

 the earthy matter being deposited in very small quan- 

 tities. Although this difference is not wholly correlative 

 with the groups in which it principally obtains, as for 

 instance, in the genus Palcemon, in which the crust of some 

 species, as the common prawn, has scarcely any earthy 

 matter, whilst in others, it is almost as solidly calcareous 



