23 



body, a set being placed in each articulation or ring. The nervous 

 system is not wholly considered. In the vertebrates a perfect system of 

 respiration and circulation, the complicated digestive process, together 

 with the number of s})ecial senses, are all distinctive characters which 

 leave no doubt about placing this group at the head of the list. Of the 

 relative position of the next two groups Cuvier was uncertain. They 

 were not placed one after another, but side by side, the moUusks having 

 a slight precedence. This he allowed, as in them he considered the 

 circulation in closed vessels was more perfectly performed, i-espiration 

 approached nearer the higher type, and the digestive organs were nearly 

 as complex as in the vertebrates. The articulates he thought lower, as 

 modifications in the vital processes became evident, respiration was not 

 performed by a single organ, but by a series of chambers scattered 

 throughout the body, and circulation and digestion were also performed 

 on a more simple plan. Modern observers however have altered this, 

 placing the articulates decidedly in front. Their external skeletons — 

 the bilateral symmetry of all parts — the distinct members for locomo- 

 tion, and the superior power of directing their movements, place them 

 in a higher scale. The mollusks, on the other hand, although having 

 greater developments of some structures, are otherwise more lowly 

 constituted, their irregularity of structure and want of symmetry 

 causing them to approach the radiates. The last group was easily 

 placed at the foot of the link. Their want of special sense, their 

 simplicity of structure and function, make it difficult to distinguish 

 many of them from the vegetable kingdom. In the formation of classes 

 the same general characters rule, whilst in addition certain characteristics 

 of the group are considei-ed. With the vertebrates, although the 

 various functions are studied, the classes are really formed on the same 

 plan as in the Linnpean system. The articulate classes depend on external 

 •characters chiefly. The arachnids are highest as they have pulmonary 

 respiration; and are furnished with eight legs. Insects have head, thorax 

 and abdomen, legs six in number. Crustaceans have their bodies 

 divided into thorax and abdomen, and legs more numerous. Annulata 

 are composed of numerous rings, legs rudimentary and no proper 

 division into head, thorax or abdomen. The cephalopods are the 

 highest of their group, having many resemblances not only to the 



