302 INVERTEBRATE MORPHOLOOT. 



ing to the eye from the cerebral ganglia pass through an optic 

 ganglion lying beneath the optic sac and are distributed to 

 the sensory cells, and the centre of the sac is filled up by a 

 cuticular mass which serves as a refractive lens (Z). In some 

 forms {Patella, etc.) the development of the eye ceases while 

 it is still in the cup form, there being then no formation of 

 corneal layers and no central lens, though the retina is usu- 

 ally covered by a thin cuticular layer. In some species of a 

 peculiar genus of the air-breathing Gasteropods, Onchidium, 

 eyes are developed upon the dorsal surface of the body, the 

 shell being lacking and the visceral hump undeveloped. In 

 structure these eyes differ very materially from those usually 

 occurring in the Gasteropoda and will be described later 

 (p. 318). 



Otocysts are usually imbedded in the tissues of the foot 

 close to the pedal ganglia, though in all cases they receive 

 their innervation from the cerebral ganglia ; they have the 

 usual sac-like form and are lined with sensory hair-bearing 

 cells and contain otoliths. 



The nephridia of the Gasteropods are in nearly all cases 

 modified from the original typical condition in accordance 

 with the asymmetry of the body (Fig. 133, re). In only a few 

 forms, so far as known {Fissurella, Patella), are two functional 

 nephridia, opening on the one hand into the pericardial 

 cavity and on the other to the exterior through the mantle- 

 cavity, perfectly developed. In other forms, such as Uali- 

 otis, Turbo, etc., both nephridia are present and are struct- 

 urally perfect, though the left* one has lost its secretory 

 function, but in the majority of cases the left (or, in forms 

 with a left-handed coiling of the visceral hump, the right) 

 nephridium is completely aborted. 



The Gasteropods are in some cases bisexual, in others 

 hermaphrodite. The reproductive sac (Fig. 133, t) is quite 

 unconnected with the pericardial enterocoel and is an un- 

 paired structure lying in the visceral hump. The ova and 

 spermatozoa in most cases reach the exterior by a special 



* The terms left and right refer to the position of the nephridia as they are 

 supposed to have been arranged in the primitive symmetrical Gasteropod. 



