CHAP. I. PRIMARY DIVISIONS OF MOLLUSKS. 5 



the second class, or the Radiata, the nerves are ar- 

 ranged in a radiated or star- like manner ; while in the 

 testaceous Mollusca, or Testacea, the nervous system 

 resides in four ganglions, either united in pairs, or 

 altogether, which are embraced by a medullary collar. 



(6'.) It would thus appear that the three great divisions 

 of Mollusca are as much distinguished by their ana- 

 tomical peculiarities, drawn from their nervous system, 

 as they are by their external and more obvious cha- 

 racters. It is by these latter, however, that they are 

 made intelligible to the generality of readers. Every 

 one, therefore, will be at no loss to understand the ex- 

 ternal distinctions of these primary divisions : 1. The 

 Testacea, or shell mollusks ; 2. The Radiata, or 

 radiated mollusks ; and, 3. The Acrita, or polypes. 

 The first being represented by shell-fish ; the second, by 

 starfish, sea eggs, and medusas ; and the third, by corals, 

 polypes, and other plant-like animals. We shall now, 

 in the first place, point out the manner of their con- 

 nection to the higher classes of animals, with the object 

 of showing that they form an integral part of that vast 

 circle which comprehends the whole of the animal 

 world. 



(7-) The primary distinction of vertebrated animals 

 is, their being possessed both of an internal column, or 

 back-bone, and of legs or other members which give them 

 the power of locomotion : it follows, therefore, that if 

 we found among the soft Mollusca any animals which 

 really possessed these characters, however slightly de- 

 veloped, we should consider them as forming the pas- 

 sage from one class to the other ; that is, from the Ver- 

 tehrata to the Mollusca. Now, there is a whole tribe 

 of animals which really present us with such a struc- 

 ture, and yet, in all other respects, they are so unlike 

 the Vertehrata, that every writer places them with the 



lysed. Between the Doridm and the Branchiopoda, there must be, or have 

 been, very many intervening forms : and we begin to think that Cuvier 

 may not be very wrong in placing many, if not all, of his naked Acephala 

 close to the Branchiopoda. But the analysis of the Acrita can alone de- 

 termine this question. 



B 3 



