ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 23 



A pedal penis, as in most Taenioglossata and Stenoglossata ; a cephalio 

 penis, as in Neritidas, Paludinidse, and Calyptra3ida3 ; a dorsal penis, 

 innervated from the subintestinal ganglia as in the Cyclostomidee and 

 Bythinia ; and a pallial penis, as in the Ampullariidae. With the ex- 

 ception of the Neritidse all the Diotocardata hitherto examined have 

 been found to be without a penis. 



Among the Pulmonates the torsion of the body displaces the organs 

 or modifies the asymmetry of the nervous system ; but among the 

 Prosobranchs it is not so ; for the dextral AmpullariidEe have the anus, 

 the penis, the gill, and the rectum to the right, and the siphon and the 

 false gill to the left ; it is exactly the same in the sinistral forms, and 

 they have the nervous system twisted in just the same way as that of 

 the dextral forms. In the Prosobranchs, then, the torsion of the body 

 does not displace the organs or modify the asymmetry of the nervous 

 system. We must, therefore, reject all the hypotheses which explain 

 r the torsion of the nervous system by that of the body. 



In Prosobranchs the presence of a lung is no indication of a relation- 

 ship between pulmonate forms ; the Cyclophori, which are always placed 

 near the Cyclostomata, are much closer to Turbo or Delphinula. 



After indicating the various modifications undergone by different 

 parts of the digestive system, M. Bouvier points out that the Proso- 

 branchs, which have become adapted to a special mode of life, have, as a 

 rule, undergone profound and apparently abnormal changes in their 

 organization ; in their progressive evolution the members of the group 

 have gone through three chief stages. The nervous system was at first 

 dialyneurous, diffused, and provided in the foot with ganglionated 

 scalariform nerve-cords ; the gill was bipectinate ; the heart, with two 

 auricles and a ventricle, was traversed by the rectum ; the very well deve- 

 loped buccal mass was situated behind the nerve-collars ; the salivary 

 glands were applied to the buccal mass, and their ducts did not traverse 

 the nerve-collars ; there was no siphon, or penis, and the renal organ 

 opened by a tube into the pallial cavity. In the second stage the nervous 

 system was dialyneurous or zygoneurous, and more or less concentrated ; 

 there were no scalariform cords in the foot ; the gill was monopectinate, 

 and a false gill more or less developed ; the heart had but one auricle, 

 and the ventricle was not traversed by the rectum ; the buccal mass 

 moderately developed, and situated in front of the nerve-collars ; the 

 salivary glands were separated from the buccal mass, and the ducts 

 traversed the nerve-collars ; a penis was generally present, the renal 

 organ opened by a cleft at the base of the pallial cavity ; the otocyst had 

 one or more otoliths, and the buccal ganglia were applied against the 

 buccal mass. The characters of the third stage are a zygoneurous, 

 highly concentrated nervous system, no scalariform pedal cords ; gill 

 monopectinate, well developed, bipectinate false gill ; heart with one 

 auricle and untraversed ventricle ; poorly developed buccal mass, from 

 which the salivary glands — whose ducts traverse the nerve-cords — are 

 separated ; buccal connective very short, but deep ; siphon, penis, pro- 

 boscis, unpaired special gland ; renal organ opening by a cleft at the 

 base of the pallial cavity ; a single otolith in the otocysts. 



These characters appear to be sufficient to justify the establishment 

 of three great divisions of the Prosobranchiate Gastropods, the Dioto- 

 cardata, tamioglossate Monotocardata, and stenoglossate Monotocardata ; 

 and this mode of classification is supported by the facts of palaeontology, 



