ANIMAL KINGDOM. 239 



have their branchiae uncovered, both have necks, and of all 

 known Gasteropoda, Tethys is the genus which has neither 

 tooth nor tongue to the mouth, forming thus an excellent 

 prototype of what will be found to exist in Clio. The 

 situations of the liver and of the intestinal canal form 

 another proof of this affinity ; and of the truth of the remark 

 of the great naturalist who has said that the genus Clio 

 u est sans contredit voisin des limaces, des doris et des autres 

 gasteropodes." It is worthy of attention also that the bran- 

 chiae of the Nudibranches are made use of in swimming, 

 like the fins of Clio. From Clio, according both to Cuvier 

 and Lamarck, we pass to the Pteropoda with an indistinct 

 head, such as his genus Hyale, and so by means of the 

 Brachiopoda return to the Acephalous Testacea. 



It is however absolutely necessary to state here the 

 difficulties which I have encountered, but have by no 

 means been able to surmount, in the above arrangement 

 of the Gasteropoda ; difficulties which no doubt have 

 their origin as much in my ignorance of M alacozoology 

 as in the little general progress hitherto made in the 

 knowledge of the innumerable species of Mollusca which 

 inhabit the depths of the ocean. Still as the object of 

 the naturalist is not to skim over difficulties but to dwell 

 on them, not to propound systems so much as to state 

 facts, I shall make a few observations on the Gastero- 

 poda of Cuvier. These animals are the most perfectly 

 constructed of the Mollusca, whether we consider their 

 nervous system, their organs of sense and locomotion, 

 the peculiarities of their structure for the purposes of cir- 

 culation and respiration, or finally the marked distinction 

 between their sexes. But on the other hand nothing can 

 be more vague than the characters by which this numerous 



