CRUSTACEA. 73 



moving along the .firm ground. In the Crab and Lobster 

 tribes, the external layer of the integument is hardened by 

 the addition of earthy particles, consisting of the carbonate, 

 with a small proportion of the phosphate, of lime."* In the 

 smaller species it is more flexible, resembling the textui'e of 

 horn or parchment. 



Distribution. — The Crustacea are universally diffused, not 

 only throughout the ocean, but through ponds, lakes, ditches, 

 and running waters. In the polar seas they are found in great 

 abundance, though the number of species is very limited. In 

 the equatorial regions, Avhile they are no less numerous, they 

 present a greater diversity of form, attain a larger size, 

 and exhibit, m the highest perfection, those peculiarities of 

 structure by which the several groups are characterised. But 

 though "the world of waters is their home," they are not 

 confined within its boundaries, for there are some species 

 which are occasional visiters to the land, and others which 

 make it their permanent residence. 



Form. — Their figures, when most faithfully delineated, 

 present a variety of form so great that at first sight they 

 seem in some cases to be the offspring of a fantastic fancy, 

 rather than the correct delineation of living animals. We 

 find legs so fonned as to do the work of jaws {Fig. 56 — 60); 

 others so constituted as to perform the function of gills; 

 while some are so long and so slender that, were we to judge 

 merely from appearance, they would seem quite disproportioned 

 to the size of the body to which they are appended. 



Characteristics As, in the radiated animals, we found the 



radiated structure most apparent towards what may be con- 

 sidered the centre of the group, so here we may point to the 

 Crustacea as examples of the complete development of the 

 jointed or articulated structure. In them we find the re- 

 spiratory apparatus existing as branchiie or gUls, however 

 varied its position or arrangement. The sexes are distinct^ 

 and all the individuals are free and locomotive. " It is the 

 combination of branchiae with jointed limbs and distinct 

 sexes which constitute the essential characters of the class 

 Crustacea.''* 



Integument. — As the integument is inelastic, and does not 

 admit of enlargement to suit the growth of the animal, a 



• Lecturea, page 163. 



