THE GASTROPODA 119 



it is inside the pulmonary cavity (Fig. 174) ; in other forms with an 

 aerial lung it is external (Fig. 89, os). 



Cyathiform bodies or gustatory bulbs, formed of taste-cells, are 

 found on the lateral and ventral surfaces of the buccal cavity in 

 sundry Rhipidoglossa, and at the sides of the buccal aperture in 

 some Heteropoda. Analogous bodies have been found on the 

 epipodial tentacles of Rhipidoglossa. 



The Otocysts, or statocysts, are hollow spherical vesicles, whose 

 internal walls are lined by a ciliated epithelium containing sense- 

 cells. These vesicles contain a liquid secreted by their epithelial 

 walls, and in this liquid are calcareous auditory concretions of 

 crystalline structure. There is a single large and spherical con- 

 cretion or otolith in the more specialised Tectinibranchs and in a 

 few adult Opisthobranchs (Lobiger, the Elysiomorpha, Hedylidae, 

 Pseudovermis, Fiona, and sundry Eolidomorpha), (viz. the Tergipedi- 

 nidae, Capellinia, Eolidiella,, Eolis aurantiaca and E. olivacea, Galvina 

 picta and G. exigua). There are numerous and usually ovoid and 

 elongated concretions, called Otoconia, in the Aspidobranchia (except 

 Bathysciadium), in some of the less specialised Taenioglossa, such 

 as Paludina, Ampullaria, Cydophorus, Valvata, Nassopsis, and the 

 majority of the Melaniidae, and in the Euthyneura in general, 

 with the exception of the Opisthobranchs mentioned above. Oto- 

 conia sometimes coexist with an otolith in certain Cerithiidae, 

 Turritella, Doto, and Oncidium, but in all larvae there is only a single 

 otolith (Fig. 116, A, III). Otocysts are absent in the adult Fermetus 

 and in some Janlhina. In creeping Gastropods the otoliths are 

 situated in the foot, in the neighbourhood of the pedal ganglia (Fig. 

 93, V), and are often adherent to these nerve-centres. In swimming 

 Gastropods, such as Heteropoda, Phyllirhoe, and Glaucus, they show 

 a tendency to approach the cerebral centres, and the same tendency 

 may be seen in the majority of Nudibranchs. In all cases the 

 otocysts are innervated from the cerebral ganglion, as may be most 

 clearly seen when they are at some distance from the pedal ganglia 

 (Figs. 123, C, ot; 142, u; 146, oi). The neuro-epithelial elements 

 are concentrated in a macula acustica, placed opposite the expansion 

 of the otocystic nerve, in the otocyst of Heteropoda. 



Cephalic eyes exist in almost all Gastropods, and there are, in 

 addition, pallial eyes in certain Oncidiidae. The two cephalic eyes 

 are situated on the tentacles, in the Euthyneura on the posterior pair 

 of tentacles. In the Streptoneura these eyes are placed at the outer 

 side of the base of each tentacle, and are borne on tubercles (Fig. 

 44, A, c) which may fuse with the tentacles, and thus, in a number 

 of instances, give the eyes the appearance of being placed half-way 

 up the tentacles, as may be seen in the Potamidae among the Ceri- 

 thiidae, in Cypraea, many Rachiglossa, certain species of Conus (Fig. 

 144, V), and Pleurotoma : in the last named they are very near the 



