22 SUMMARY OF CURRENT RESEARCHES RELATING TO 



(C) Left * Zygoneurou8 Nervous System : Anipullariidao, some 

 Crcpitlulidae, Naticidao, Lamellariidao, Cypnoida3. 



(D) False Orthoneurous Nervous System : Helicinidao and Neritidao. 

 Right zygoneury becomes more marked as one ascends the scale of 



Prosobranchs ; the right pallial anastomosis of the Aspidobranchs is at 

 some distance from the right ganglion. In Paludina, Littorina, and 

 Cyclostoma, tho two pallial nerves fuse in the walls of the body. Among 

 the Cerithiidae, Melauiidae, and Cyprreidae, there aro some genera more 

 or less dialyneurous, and others which are more or less distinctly 

 zygoneurous. 



Once right zygoneury is realized, tho right anterior pallial nerve 

 becomes a connective ; this is generally pretty long, but in most of the 

 Stenoglossata it is so short that the subintestinal ganglion becomes 

 intimately connected with tho right pallial ganglion. 



The nervous system of Diotocardata is essentially characterized by 

 the diffusion of the nervous centres. From the point of view of the 

 nervous system there is no solution of continuity among the different 

 groups which compose the order of Prosobranchs. Thus, in the Tsenio- 

 glossata the Ianthinidae and tho Ampullariidas have a very long cerebroid 

 commissure ; the Ampullariidae, Paludinidas, Cyclophoridae, &c, have a 

 labial process and a labial commissure, and the AmpullariidaB and the 

 Ianthinidae very long lateral connectives. 



The successive transitions between the Diotocardata and the Monoto- 

 cardata are more sharply indicated by the ganglionic cords of the foot ; 

 the buccal ganglia also undergo progressive modifications as one ascends 

 in the order, for in Halia and the Purpuridae they are closely ap- 

 proximated and almost concentrated into a single mass. 



Other modifications are presented by the cerebral commissure, and the 

 maximum of concentration is exhibited by the Stenoglossate Monoto- 

 cardata, where the buccal ganglia are very close to the cerebral ganglia, 

 and very far from the buccal mass. With these variations there cor- 

 respond changes in the relations of the buccal connectives. 



In the most primitive types the anterior part of the mantle is almost 

 symmetrically and solely innervated by the pallial ganglia. If the 

 right gill and false gill are absent, there is no subintestinal ganglion, 

 and its position in the commissure or in its vicinity is simply indicated 

 by one or two right pallial filaments. As one ascends the Taenioglossata 

 the asymmetrical innervation of the mantle increases in importance, 

 especially on the right side. The Diotocardata are the least asymmetrical 

 of all the Prosobranchs. 



After describing the innervation of the gills, and the characters of 

 the visceral ganglia, the author proceeds to consider the otocysts ; these 

 may be divided into three groups ; ( 1) Otocysts with numerous otoliths 

 as in Diotocardata and some Rostrifera ; (2) Otocysts with numerous 

 otoliths inclosing a large round otolith, as in Turritella rosea ; and (3) 

 Otocysts inclosing a single otolith, as in remaining Prosobranchs. 

 Although it would be an error to deny all systematic value to the oto- 

 cysts, the author thinks that their importance, from this point of view, 

 has been over-estimated. 



The penis is not always, as has been stated, a cephalic formation 

 innervated from the cerebral ganglion, for four kinds may be distinguished. 



* In the text B and C are both "Systeme nerve ux zygoneure a droite." 



