ORGANS OF SENSE. T5 



Cyprsea, whicli lobes are thrown over the back of the shell so as 

 nearly to cover it (Ixi, 99). The anterior lobes of the foot 

 existing in man}^ moUusks, as in Buccinnm and Harpa for 

 example, may also be regarded as tactile in function. 



In the nudibranchiates, besides the various appendages or 

 prolongations of the mantle-border, there are dorsal tentacles 

 (rhmophores, vi, 19), situated near the head, and certainly 

 organs of touch. 



The Pulmonata or snails are provided with very sensitive 

 tentacles (vi, 12), those of the terrestrial species being invagi- 

 nate and combining in addition the organs of vision and possibly 

 of smell ; while the aquatic species, Physa, etc., have the eye 

 usually separated from the tentacle. Glandina, which is one of the 

 few carnivorous Pulmonata, has remarkably developed labial 

 palpi, resembling enormous moustaches (vi, 12). These palpi, 

 says Fischer, are in ceaseless movement whilst the animal is 

 crawling ; and when it approaches another mollusk (Bulimulus or 

 Helix), these organs are applied first to the shell, then to the body 

 of the victim, which is soon drawn, in part, across the extremely 

 dilated mouth of the Glandina. A very large nerve proceeding 

 from the cerebral ganglions is distributed through the labial 

 palpi, the sensibility of which must be extremely delicate. 



Pelecypoda. The fringed border of the mantle is very quickly 

 retracted wherever touched. In the Pectens and Limas, which 

 swim rapidl}?-, these fringes are greatly developed. In Unio and 

 Anodonta they are so sensitive as to perceive the slightest 

 movement of the water in which they exist. The extremity of 

 the foot, in the Unionidse is also very impressionable (Baudon). 



Organs of Sight. Cephalopoda (vi, 10,11). The two sessile eyes 

 are lodged in orbital cavities on either side of the head, in the 

 dibranchiates ; in the tetrabranchiates they are elevated upon 

 peduncles. In the former case the cephalic cartilage, as we have 

 already seen, partly encloses them, whilst in some instances 

 special orbital cartilages are also present ; the enclosure is 

 completed by a fibrous capsule continuous with the cephalic 

 cartilage, which becomes transparent over the eye and is likened 

 to the cornea or even to the eyelid in vertebrates. This trans- 

 parent capsule presents several modifications ; it may be entire 

 or with a small perforation as in the Octopods, in Sepia, Loligo 

 and other genera, constituting the division Myopsidse of 

 d'Orbigny ; or it may have a wide opening, for the projection of 

 the crystalline lens, as in Ommastrephes, Loligopsis, etc. — 

 the Oigopsidde of d'Orbigny : it is entirely wanting in Nautilus. 



We find in the dibranchiates a large portion of the eye-chamber 

 occupied by the optic ganglion, by ocular muscles and by a 

 white glandular substance. The silver}^ tapetum lines but does 

 not adhere to the ocular capsule ; its two layers pass into one 



