318 INVERTEBRATE MORPHOLOGY. 



tic forms, and the Stylommatophora, which includes the ter- 

 restrial forms, and Onchidium. 



This last genus in addition to the usual eyes borne upon the tentacles 

 is in some species further provided with a number of eyes situated upon 

 the back and differing from the typical eye in the arrangement of the retinal 

 cells. As has been seen, the optic nerve in typical eyes on entering the eye 

 spreads out in a layer to form the retina, the terminal optic cells being 

 situated on that surface of the retina which is turned towards the light. 

 The dorsal eyes of Onchidium, however, present a somewhat different 

 arrangement, the cells in which the nerve-fibres tei initiate having their 

 distal ends turned away from the light, which to affect them must pass 

 through the layer of nerve-fibres formed by the spreading out of the optic 

 nerve. Compared with the retinae of typical eyes, those of the dorsal eyes 

 of Onchidium are inverted and have assumed an arrangement exceedingly 

 rare in Invertebrates, but typical for the lateral eyes of the Vertebrata. 



Otocysts are always present, and the tentacles borne by the 

 head are probably tactile in function. In the Stylornmatoph- 

 orous Pulmonates there are in some cases (Helix) two pairs 

 of such tentacles, the eyes being situated upon the posterior 

 pair, both pairs furthermore being capable of being invagi- 

 nated for protection into the body-cavity, a peculiarity not 

 presented by the tentacles of the Basomrnatophora. As 

 stated above, the osphradium is represented in certain aquatic 

 forms, but in the Stylommatophora it has disappeared with 

 the suppression of the ctenidiuni. 



The Pulmouata are hermaphrodite, the epithelium of the 

 reproductive gland (Fig. 143, hg) differentiating into both 

 spermatozoa and ova, there being no localization of the for- 

 mation of either one or the other in a special portion of the 

 gland, as happens in some Opisthobranchs. In the Basom- 

 matophora and certain terrestrial Pulmonates, such as Vagi- 

 nula and Onchidium, the common duct (hd) for the spermatozoa 

 and ova divides and passes to the exterior by two distinct 

 and separate apertures. Thus in Limncea the hermaphro- 

 dite duct shortly after leaving the gland divides, and into one 

 of the branches immediately after the division there opens a 

 well-developed albuminiparous gland (cd\ and it then becomes 

 somewhat folded, forming what is termed the uterus (w). 

 Beyond this structure the duct, now known as the oviduct (od), 

 receives the duct of a nidameutal gland and dilates into a 



