THE SNAIL 79 



very short course, enter a large pair of visceral ganglia, and 

 are continued beyond the visceral ganglia into a large median 

 abdominal ganglion. The whole system of visceral and ab- 

 dominal ganglia with the nerves connecting them and the 

 pleural ganglia is known as the visceral loop. The visceral 

 ganglion of the right side gives off a long nerve to a ganglion 

 underlying a small patch of sensory epithelium (osphradium) 

 in the respiratory chamber. This is the osphradial ganglion. 



In the snail the cerebral and buccal ganglia have much the 

 same relations as in Limnaea. The connectives passing round 

 the oesophagus to the subcesophageal mass are double, and 

 represent the pleural and pedal connectives. The subceso- 

 phageal mass is divided into an upper and a lower portion by 

 an artery passing through it. The lower portion represents the 

 closely opposed pedal ganglia from which stout nerves pass to 

 the foot. The upper portion represents the fused pleural, 

 visceral and abdominal ganglia. Nerves are given off from it 

 to the viscera, and a specially stout nerve passes to the genera- 

 tive organs (fig. 18, A). 



In Helix the visceral loop is obsolete, in Limnaea it is short, 

 and the paired ganglia are symmetrically arranged. Such a 

 disposition is called euthyneurous (orthoneurous by Continental 

 authors). But in Paludina the visceral loop is long, and has 

 been involved in the rotation of the visceral hump to such an 

 extent that it is twisted into a figure of eight, the originally right 

 visceral ganglion lying to the left and above the gut, the original 

 left visceral ganglion lying to the right and below the gut (fig. 

 1 8, C). This disposition of the visceral nerve cords is known 

 as streptoneurous (foreign authors chiastoneurous). The uni- 

 valve gastropods are divisible into two well-marked orders 

 according as they are streptoneurous or euthyneurous, and 

 Paludina and Helix may be taken as examples of the two groups. 



The eyes of the snail can only be studied by means of 

 sections. They are highly developed optical organs, each con- 

 sisting of a hollow vesicle or eyeball lined by an epithelium 

 which is transparent in front, but modified in the posterior part 

 of the vesicle to form a number of retinal cells embedded in 

 pigment. The retinal elements are turned towards the cavity 

 of the vesicle, and the optic nerve does not perforate the retina 

 as in the vertebrate eye, but spreads out on the posterior sur- 

 face of the eyeball. The cavity of the eyeball is occupied by 



