CLASSIFICATION OF THE ANIMAL KINGDOM. » 125 



principally marine in their habits, but they are also found abundantly in 

 our lakes and ponds. Their outward appearance resembles that of insects 

 in structure, being composed of three parts — a hard cuticular secretion or 

 shell of a calcareous nature, a coloured pigment, and a vascular dermis. 

 These animals, in their lowest form, correspond with the condition of the 

 skeleton met with in the Myriapoda and in the larva of many insects, 

 "the whole body being composed of a series of similar segments, to which 

 are appended external articulated members of the simplest construction," 

 and we shall find, as we examine the more organized tribes, that there 

 is a gradual concentration of these segments, and a consequent corresponding 

 coalescence of the nervous system within. The quantity of rings in the 

 body of each species is supposed to be the same, the normal number being 

 twenty-one, seven of which belong to the head, and seven each to the 

 thorax and abdomen; and, in illustration of this theory, Professor Jones 

 brings forward the Talitra, as having the seven cephalic segments all 

 united, but their existence indicated by seven several pairs of appendages, 

 while the seven thoracic and abdominal segments are all distinct; the 

 Lobster, (Astacus marinus,) as having the fourteen cephalic and thoracic 

 rings all joined together, but the abdominal segments still distinct; and 

 Crabs, as being still more united; and one, the King Crab, (Limulus 

 Polyphemus,) as having the division of the abdomen also obliterated; and, 

 as he well observes, we cannot but trace, as we review this comprehensive 

 class from the lowest to the highest types, the same steps "whereby we 

 pass from the Annelidans to the Myriapods, and from thence to the insect, 

 the Scorpion, and the Spider." 



The Decapod division of this class is alone noticed in the Professor's 

 work, and it is divided into three extensive families — the Macroura or 

 swimmers, the Anomoura or Hermits, and the Brachyura; of which last 

 the Common Crab is a familiar example. Of the first of these groups 

 the best known is the Common Lobster; it has five pairs of articulated 

 limbs on each side of its mouth, used for the prehension of food, which 

 are called foot-jaws. The next pair of legs succeeding to these are very 

 remarkable in structure; they are thick and muscular, and are armed with 

 chehe or claws, one pair being provided with sharp teeth for tearing prey, 

 and the opposite with large blunt tubercles, for holding fast to any sub- 

 marine matter. After these come four pairs of slender legs, the two first 

 having also a pair of feeble forceps. These last pairs of legs are but little 

 used for locomotive purposes, as the Macroura always employ their tails 

 in swimming, and if the former were more powerful, they would necessarily 

 hinder the action of the latter. 



The next great family, the Anomoura, or Soldier Crabs, have the 

 hinder part of the body soft and coriaceous, which they protect by forcing 



