ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 21 



ings in its organ. The rich innervation of the two buccal lobes is 

 described. Other nerves supply the lips proper. 



The pedal centre has really a double commissure. Each ganglion 

 gives off three large nerves below and three above. The largest and 

 most internal of the latter innervates the sensitive region of the foot 

 below the labial palps the other two go to the portion of the foot in 

 front of the head. Of the inferior pedal nerves, the two medians supply 

 the middle region of the foot, the two outer pass outwards to the two 

 large lateral lobes which ascend dorsally, and are sometimes erroneously 

 called the mantle. The intermediate pair innervate the most external 

 portion of these same lobes. 



The asymmetric centre. — From the two little ganglia which lie on the 

 pedal centres, and belong to the oesophageal collar, there rises on each 

 side a cord which passes to the neighbourhood of the heart and the base 

 of the gill. There the couple unite in two closely adjacent ganglia. A 

 long closed loop forms with the two superior ganglia the transversal 

 chain of the asymmetric centre. 



From the first ganglion on the left, near the pedal of the same side, 

 a nerve descends to where the mantle properly begins, and there divides. 

 The two precardial ganglia give off two large nerves, which are dis- 

 tributed on mantle and viscera. The details of their distribution and 

 the nature of the branchial ganglion are noted. The nerves of the neck 

 arise from the dorsal surface of the pedals. 



It is important to notice that mantle, viscera, and gill are supplied 

 as usual by the asymmetric centre, the median ganglia of which are far 

 separated from the collar, and in the cardiac region. They are united 

 by a long connective-like commissure. The mantle-like lobes of the 

 foot are innervated from the pedal ganglia. 



Nervous System of Prosobranchs.* — The following are some of the 

 more important general conclusions reached by M. E. L. Bouvier. 

 The nervous system of Prosobranch Mollusca is characterized by a 

 crossed visceral commissure, which is only wanting in the orthoneuroid 

 Azygobranchs. Except, perhaps, in the Docoglossata, there are also two 

 pallial anastomoses ; the right anastomosis is related to the right 

 pallial nerve which arises from the pallial ganglion of the same side, 

 and with another right pallial nerve which arises from the subintestinal 

 ganglion, or (when that ganglion is absent) from the subintestinal branch 

 of the visceral commissure. The left anastomosis is established between 

 the left pallial nerve, which arises from the left pallial ganglion, and a 

 branchio-pallial nerve which is given off from the subintestinal com- 

 missural branch. 



If the right pallial nerve passes by the subintestinal ganglion before 

 passing to its area of distribution, the nervous system is zygoneurous to 

 the right, or there may be zygoneury to the left ; in all other cases the 

 nervous system is dialyneurous. Eight is much more frequent and 

 important than left zygoneury. We may classify the Prosobranchiata 

 thus : — 



(A) Dialyneurous Nervous System : Chiastoneurous Diotocardata ; 

 Holostomatous Proboscidifera ; the majority of the Eostrifera. 



(B) Eight Zygoneurous Nervous System : Siphonostomatous Pro- 

 boscidifera ; Stenoglossata ; some Eostrifera. 



* Ann. Sci. Nat.— Zool., iii. (1887) pp. 1-510 (19 pis.). 



