46 SHELLS AND SHELL-FISH. PART I. 



(38.) We may noAv'^ commence a rapid survey of 

 the course by which the class before us resolves itself 

 into one great circle, the furtner details of which will 

 be noticed under the several chapters devoted to the 

 chief divisions. We have already expressed our con- 

 viction that the germs of the Testacea exist among the 

 various discordant animals comprising Cuvier's Paren- 

 chymata, but it is by no means clear which is the pre- 

 cise point where the very first indication of this struc- 

 ture is seen ; this cannot be cleared up until the Acrita 

 and the Radiata, not to mention the annulose Vermes, 

 have been sufficiently analysed. Without speculating, 

 therefore, on the probability or possibility of such 

 genera as Echinorynchus and Herula being the rudi- 

 mentary type, we may safely point to the Tremadotes 

 of Rudolphi, and the, whole of the second family of 

 Cuvier's Parenchymata, as containing the vermiform or 

 most aberrant types of the class before us. These, in 

 fact, are nothing more, in one sense, than naked gas- 

 teropod Mollusca, crawling upon their bellies, or ad- 

 hering by means of cup-shaped suckers, which are 

 analogous to, and perform the same office as, the single 

 disk of the more perfect testaceous gastropods. Some 

 of these, like the FasciolcB of Linnaeus, are feeders on 

 the internal parts of animals, and adhere, by means of 

 these suckers, to the viscera of quadrupeds, birds, and 

 fish ; others, as Cyclocotyles of Otto, are external para- 

 sites ; while the true PlanaridcE are entirely free^ and 

 swim about in the water. Now, it is from these 

 animals that nature throws out, as it were, two lateral 

 branches. One of these, emanating from the Planaridce, 

 conducts us, in the most beautiful and graduated man- 

 ner, to the NudibranchUi ; the other, departing from 

 the Fasciola, leads us, by means of such genera as 

 Menostoma of Blainville, and Hectocotylcs of Cuvier, to 

 the no less vermiform animals among the Ftrolcp and 

 other finless Hetcropoda. This latter branch, however, 

 we shall leave after this indioation, and pursue the for- 

 mer. Commencing, then, with the Planarida', we see a 



