NERVOUS SYSTEM 269 



have fused, although in the swan mussel (p. 280) they are separate. 

 Nerves from the cerebral ganglia go to the mouth and anterior 

 sense organs, and from the other ganglia to the parts of the body 

 suggested by their names. The eye, at the end of the longer 

 tentacle, has a lens-like structure and a retina ; it is especially 

 good at detecting quick movements, as is easily shown if a hand 

 is moved across its field of view, but the range of vision is small. 

 There is a pair of statocysts near the pedal ganglia, and the 

 tentacles are highly sensitive to smell. 



REPRODUCTION 



The snail is hermaphrodite, and is peculiar in producing both 

 types of gamete in the same gland, which is therefore called an 

 ovotestis. It is a whitish lobed structure at the top of the visceral 

 hump. From it there leads a short coiled hermaphrodite duct, 

 and this passes into a longer common duct, which runs forward. 

 It is incomxpletely divided into male and female channels. At 

 the junction of hermaphrodite and common ducts is a large 

 albumen gland. The common duct finally divides into a left 

 vas deferens and a right oviduct or vagina. The vas deferens 

 leads into a muscular penis ; just before it does so there is given 

 off a blind diverticulum, the flagellum, which runs back alongside 

 the common duct. The oviduct also gives off a long blind diver- 

 ticulum or spermatheca, which may be recognised by the sub- 

 spherical expansion at its end. (This is sometimes called the 

 receptaculum seminis, but this name is better reserved for another 

 structure to be mentioned below.) In H. aspersa, but only rarely 

 in H. pomatia, the spermatheca itself has a diverticulum, which 

 in dissection is easily confused with the flagellum. Below the 

 spermatheca there opens into the oviduct a lobed mucous gland 

 and below this again the dart sac. Oviduct and penis open together 

 at the common genital aperture. Sperms are produced during 

 most of the year, and are bound together in packets <K sperma- 

 tophores by the secretion of the flagellum. When copulation is 

 about to occur two snails cruise round each other in decreasing 

 circles until, when they are nearly touching, the dart sacs 

 violently contract nearly simultaneously, and from each a 

 calcareous dart is shot into the other snail with enough speed to 

 penetrate deep into the internal organs. There is a pause after this, 

 and after a time the snails come into close contact, and the penis 



