MORPHOLOGY OF THE HELICINID.E. 789 



nei've- branches have been traced in sections. The subintestinal 

 trunk in Alcadia palliata^ A. hollandi, and Eiitrochatella pulchella, 

 after leaving the subintestinal ganglion, which latter is unrecogniz- 

 ably fused into the pleural centres, courses along the floor of the 

 anterior division of the general body-cavity, below the pharyngeal 

 bulb, the radular sac, and oesophagus. It keeps closer to the right 

 than to the left columellar mascle, and on reaching the posterior 

 end of the muscle it doubles the angle between it and the visceral 

 mass, and entering the latter turns to the left and enlarges to 

 form a ganglion of some size fi-om which several nerves are given 

 off. That this is a true ganglion-centre, and not a mere nodal thick- 

 ening at a point from which several nerve-branches originate, is 

 demonstrated by the considerable sheath of nerve-ganglion cells. 

 The principal nerves issuing from the ganglion, in addition to the 

 main trunk, are the following : — a small nerve, ?/, which passes to 

 the right near the surface of the visceral mass and is distributed 

 to the gonads and liver. Another small nerve, v\ which passes 

 to the left of the visceral mass and appears to innervate the right 

 lobe of the liver and' siii'face of the stomach. A stout nerve, 

 n.ge)i., which runs to the right, passes above the oviduct or 

 sperm-duct, gives off a large branch to the mucous gland, and 

 tvirns forward to break up into twigs on the posterior part of the 

 complex of genital ducts : this is the genital nerve, and it 

 identifies the ganglion from which it originates as the visceral 

 ganglion. There is no separate genital ganglion as in the 

 Neritidje. The hinder end of the visceral ganglion is continued 

 into a. rather stout nei've, which may be regarded as the 

 continuation of the main trunk of the subintestinal : this passes 

 through the liver and below the smaH and large intestine, and 

 turning towards the left it passes towards the lower border of the 

 right moiety of the kidney, near which it enlarges to form a small 

 but distinct ganglion, which I take to be the representative of the 

 elongated visceral ganglion of the -Nei-itid^e. From this ganglion 

 small nerves are given ofi' to the liver and kidney, and a larger 

 nerve passes below the kidney, skirts the uropore, and can be 

 traced beyond as far as the amicle of the heart, at wliich point it 

 ceases to be recognizable. It is a matter of exti'eme difficulty to 

 follow the above-mentioned nerves through the liver and intestinal 

 coils by simple dissection, and I have only been able to make sure 

 of their ultimate course by the study of serial sections. 



After this criticism of the general characters of the nervous 

 system, I may retui'n to the consideration of some special details. 

 The pleuro-pedal centres and with them the subintestinal 

 ganglion are, as I have already said, so intimately fused as to be 

 practically indistinguishable as separate ganglia. This fusion is 

 brought out in a striking manner in sections. Figs. 45 to 52 

 (PI. XXXVIII.) represent selected members of a series of nearly 

 transverse sections through the pleuro-pedal centres of Alcadia 

 hollandi. Fig. 45 represents a section through the pedal cords at 

 the point where they begin to diverge from one another. Dorsad 



