GASTEKOPODA. 293 



The head of the Pneiimodermon is characterised by two groups of 

 numerous tentacula, each terminated by a sucker ; and the recent re- 

 searches of Eschricht* have brought to light an analogous structure in 

 the Clio, whose cephalic organs of prehension are much more complex * 

 than Cuvier supposed. Each of the six conical retractile processes of 

 the head are perforated by numerous cavities, recognisable to the 

 naked eye as red points, but containing about twenty microscopic 

 pedunculated discs, the total number of which in the head of the 

 Clio, Eschricht estimates at 360,000 ! There are two slender and 

 simple tentacula which seem to exercise only the tactile faculty. Two 

 supra-oesophageal or cerebral ganglia are developed upon the upper 

 part of the nei'vous collar which incloses the beginning of the ali- 

 mentary canal ; the two pedial and the two branchial ganglia are 

 closely approximated and connected with the inferior and lateral parts 

 of the nervous collar. From these centres the nerves are distributed 

 to all the viscera and parts of the body. 



The male and female sexual organs are combined in the same in- 

 dividual. The duct of the voluminous testis communicates with a 

 spherical vesicula seminalis, and is then continued to the base of an 

 intromittent organ, which projects from an orifice on the right side of 

 the head; this organ Cuvier compares in the Cijmhulia to a small 

 proboscis ; in the Clio it is almost as long as the body, and proves 

 impregnation to take place by reciprocal coitus, as in many of the 

 inferior Gasteropods. 



The ovarium is also a voluminous organ ; and the oviduct is wide, 

 and provided with glandular parietes through a great part of its 

 course ; it terminates close to the base of the intermittent organ. 



The development of the ova of the Pteropoda has not yet been 

 observed. 



Class Gasteropoda. 



The transition from the Pteropoda to the present class is obviously 

 made by the Philliroe, the Glaucus, the Carinaria, and the Firola, 

 all floating pelagic genera remarkable for the delicacy of their tissues, 

 and the rudimental character of their gastric foot : these aberrant 

 Gasteropods manifest the same affinity to the preceding group by the 

 jjresence, in some, of lateral, symmetrical, more or less aliform ex- 

 pansions, and, in others, of a shell characterised by its elegant sym- 

 metry, lightness, and transparency ; that of the Carinaria much re- 

 sembles the shell of the Cymhulia in form, but is of a calcareous 

 texture. 



* Loc. cit. 

 u 3 



