282 [Oct., 1845 



An objection might be made to this system, on the ground of the 

 apparent absence of nerves in some of the lower orders. But a 

 real absence can hardly be concluded, from our inability to dis- 

 tinguish them. Many of these animals show by their voluntary 

 motions and sensibility that nervous influences traverse the body: 

 moreover, nervous matter is secreted in lines. We can therefore 

 only infer the indistinctness, and not the absence of nerves from our 

 ineffectual efforts to trace them out; and we must consequently be 

 guided by general structure, in determining the relations of groups, 

 when the nerves fail of giving aid. 



The above arrangement fails, in some respects, of present- 

 ing a clear idea of the system in nature, although highly philoso- 

 phical in its general features. A study of the animal kingdom, as 

 has been lately shown, brings to light lines or general systems of 

 development branching up from the lowest Infusoria to the higher 

 grades of life. It is not true that the forms among the lower grades 

 are actually copied in any of the imperfectly developed young of 

 the superior; yet there is some general analogy, sufficient to indi- 

 cate that the former commence on the same system of development 

 with some of the latter, although carried essentially out of the direct 

 upward line by the peculiar vital forces of the species. The Roti- 

 fera are decidedly crustacean in type. Their stout mandibles 

 are precisely those of the Cyclopidce in position, and also in 

 general form ; and in their mode of reproduction, the animals are 

 closely similar; yet no young crustacean is ever a Rotifer. The 

 latter belongs to the same[system of development with the former, 

 but is a distinct branch, from the regular line, characterized by the 

 peculiar natatory organs, which appear to be the analogues of the 

 branchial or basal appendages to the feet in Crustacea. The same 

 reasoning applies to the Bryozoa or Flustroid polyps, which are as 

 nearly allied to the Tunicata as the Rotifers to Crustacea.* It is a 

 side-development from the imaginary line, which connects the In- 



*The Bryozoa have been placed near the Rotifera ; but the absence of man- 

 dibles, as well as their peculiar type of structure, separates them widely from 

 these crustaceoid species, and allies them as closely to the Tunicata, with 

 which they were first associated by Thomson, under tho name of Tolyzoa. 

 Lister has a finely illustrated article on this subject in the Philosophical Tran- 

 sactions, for 1834, p. 365 



