ORGANS OP TOUCH. 35 



species having a simple mouth, the cesophageal ring surrounds 

 the oesophagus immediately behind the oral mass, but in those 

 having a proboscis or snout, it is situated so far back as to 

 remain at rest whilst the proboscis is protruded or retracted. 



The cerebral ganglia are placed above or at the sides of the 

 ossophagus, and from them proceed the nerves of the e} r es, tenta- 

 cles, lips and rnouth ; the pedal ganglia are under the oesoph- 

 agus, and from them the acoustic and pedal nerves arise ; the 

 visceral ganglia are mostly at the underside, somewhat above 

 the pedal ganglia, and here the nerves supplying the mantle, 

 branchiae, viscera, heart and columellar muscle take their 

 origin. Almost all of the nerve-cords arising from these three 

 ganglionic pairs may also develop ganglionic enlargements 

 whence numerous nerves in their turn originate. Such is the 

 general plan of the nervous system in prosobranchiates, differing 

 but little from that of the lamellibranchiates or bivalve mollusca. 

 A sufficiently clear idea of the modifications of the ganglia, and 

 of the origin, course and function of the various nerves, may be 

 obtained from my figures on plate 6, with the accompanying ex- 

 planations, so that it will not be necessary to particularize here. 



Organs of Touch. 



The principal tactile organs are the tentacles, but there are in 

 addition, in certain prosobranchiates lobular productions of the 

 head near the tentacles, which appear to have a similar function 

 (plate 3), whilst the whole exposed surface of all mollusca is 

 delicately sensitive. 



The tentacles, always two in number in the prosobranchiates, 

 are solid structures, not invaginate and capable of retraction 

 within the head as are those of the pulmonates : they arise from 

 the front dorsal part of the head and in the proboscidiferous 

 species are situated at the base of it. 



The tentacles usually bear the eyes upon stalks which are con- 

 nate with or branch out from them. The position of the eyes 

 varies in different genera ; thus they are found near the bases of 

 the tentacles in Littorina, Dolium, or near the middle, as in 

 Murex, Fusus, Cassis, Mitra, etc., or even at the end, as in 

 Terebra. In Strombus the robust eye-stalk originates about the 

 middle of the filiform tentacle. In many holostomates, as Tro- 



