50 SHELLS AND SHELL-FISH. PART J. 



are provided with reflected teeth,, to aid deglutition. 

 On turning to the Dithyra, however, we find a totally 

 different structure : the mouth has neither lips, jaws, 

 nor teeth, hut is " a simple aperture" entering into the 

 stomach ; the food, in short, is sucked in, and is 

 swallowed entire, as it is hy birds : so that, strange as 

 it may appear, the two most perfect groups of the 

 testaceous Mollusca exhibit, generally speaking, the 

 same mode of feeding as the two most perfect groups 

 of vertebrated animals. It seems hardly necessary to 

 strengthen analogies so beautiful ; yet we cannot omit, 

 in this place, that there are not wanting indications, even 

 in the structure of the bivalve shell-fish, to remind us of 

 the peculiar, character of birds : in these latter, the sides 

 of the body are enveloped by two broad and expansive 

 wings ; in like manner is the body of an acephalous 

 bivalve enveloped on its sides by the ample folds of its 

 cloak or mantle. Yet neither of these appendages occur 

 among the typical quadrupeds and the typical gastro- 

 pods.* But it is time to pass on to the Nudibranchia 

 and the reptiles. Do these represent each other ? and 

 in what manner ? We will set aside that forbidding 

 appearance which these reptile-looking Mollusca possess; 

 and which indicates, at first sight, some analogy to the 

 true reptiles, because this is a mere matter of opinion : 

 not so, however, is the circumstance that both these 

 classes crawl upon their belly, and are destitute of any 

 limbs. The tritons, in fact, have no real feet ; and 

 the serpents, standing at the head of the Reptilia, are 

 eminently distinguished in the very same way. The 

 analogies of the Parenchymata, in general, have been 

 sufficiently explained ; it is, therefore, only necessary 

 to observe in this place, that the Amphibia are the most 

 imperfect of the vertebrated animals, just as the Paren- 

 chymata are the most simply constructed, and the fur- 

 thest removed from the typical Testacea. Their short, 

 thick, and broad form reminds us again of the onisci- 

 form type of annulose animals. We have now only to 

 * Except in such as are aberrant in their own circles, as the Volulid<e, &c. 



