28o ZOOLOGY FOR MEDICAL STUDENTS chap. 



those of Fig. 115, B, i and 2. The retina here lines a deep cavity 

 communicating with the exterior by a minute pore (Fig. 112^ p). 



In the simpler of these types of molluscan eye there can be no 

 question of vision in the ordinary sense of the term — they distinguish 

 merely between light and shade. The rounded mass of vitreous body 

 serves in the first instance merely to render the eye more sensitive by 

 concentrating the light upon the retina^ but with the development of a 

 lens the formation of an image becomes possible and an incipient capacity 

 for short-sighted vision is acquired (Land-snails). In Nautilus the small 

 opening takes the place of a lens, as in a "pin-hole camera/' rendering 

 possible a picture of surrounding objects. And finally the highly com- 

 plicated eye of the typical Cuttlefish provides for a highly developed 

 sense of sight with power of accommodating the vision to objects at 

 varying distances. As in the case of a fish the eye is normally focused 

 for objects near at hand but special muscles are present whereby the 

 lens can be drawn closer to the retina so as to focus more distant objects. 

 The molluscs with the rarest exceptions (Chiton and its allies) possess 

 a pair of otocysts, associated as in other cases with the balancing of the 

 body in relation to gravity. As elsewhere each otocyst is formed from 

 a sensory thickening of the ectoderm which sinks below the general 

 surface and, except in the most archaic Pelecypoda, loses its communica- 

 tion with the exterior. As a rule the otocysts are situated close to the 

 pedal ganglia but as their nerve-fibres have been traced to the cerebral 

 ganglia it would appear that they belonged originally to the head region 

 and have become secondarily shifted into that of the foot, retaining 

 their primitive nervous connexion with the ganglia of the head region. 



The last type of molluscan sense-organ is the osphradium — a patch 

 of sensory epithelium in the neighbourhood of the ctenidium. It is 

 believed to have to do with testing the water that bathes the surface of 

 the ctenidium but the exact nature of this function is quite obscure. 

 The osphradium is best seen in the Gasteropoda where it is connected 

 by nerve-fibres with the visceral loop. 



While the Mollusca are typically marine animals, a considerable 

 number of Pelecypoda and Gasteropoda have taken up their abode in 

 fresh water and a number of Gasteropods (Snails and Slugs) have become 

 terrestrial. Some of these latter have in turn reverted to the aquatic 

 habit (Water-snails), retaining however for the most part their terrestrial 

 habit of breathing air by means of their lung. 



The chief direct interest of the Mollusca to medicine centres in the 

 pelecypodan habit of drawing into the body minute floating particles. 

 Disease-producing bacteria such as Typhoid bacilli drawn into the body 



