106 ZOOPHYTES. 



II. CYCLO-GANGLIATA or MOLLUSCA. Cephalopoda, Pteropoda, Gas- 

 teropoda, Conchifera, Tunicata. 



III. DIPLO-NEURA or ARTICULATA. Crustacea, Arachnida, Insecta, 

 Myriapoda, Annelida, Rotifera, Entozoa. 



IV. CYCLO-NEURA or RADIATA. Echinoderma, Acalephse, Polypi- 

 phora (zoophytes), Poriphora (sponges), Polygastrica. 



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 distinguish 

 them. Many of these animals show by their voluntary motions and 

 sensibility that nervous influences traverse the body : moreover, 

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

 infer the indistinctness, and not the absence of nerves, from our in- 

 effectual 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. 



106. The above arrangement fails, in some respects, of presenting 

 a clear idea of the system in nature, although highly philosophical in 

 its general features. A study of the animal kingdom, as has been 

 lately shown, brings to light, lines or general systems of developement 

 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 indicate that the former 

 commence on the same system of developement with some of the 

 latter, although carried essentially out of the direct upward line by 

 the peculiar vital forces of the species. The Rotifera are decidedly 

 Crustacean in type. The stout mandibles are precisely those of some 

 of the Cyclopidse, even in position, and also in general form; and in 

 certain peculiarities in the mode of reproduction, the animals are closely 

 similar; yet no young Crustacean is ever a Rotifer. The latter 

 belongs to the same system of developement with the former, but 

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

 natatory organs, which appear to be analogues of the branchial or 

 basal appendages to the feet in Crustacea. The Bryozoa,* or Flus- 



* The Bryozoa have been placed near the Rotifera ; but the absence of mandibles, 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 the name of Polyzoa. Lister has a finely illustrated article on this 

 subject, in the Philosophical Transactions, for 1834, p. 365. 



