4 SHELL GALLERY. 



fibres of the optic nerve entering the distal and not the proximal 

 ends of the retinal cells. Eyes of a similar construction are to be 

 found on the back of the shell-less Oncidium, and may be about 

 one hundred in number. 



Quite recently eyes of a remarkable character have been detected 

 by Professor Moseley on the shells of some of the Chitons ; they 

 appear to be modified from tactile organs, and are innervated like 

 the ordinary molluscan eye ; they are perhaps most remarkable 

 from their enormous number, more than ten thousand being pre- 

 sent on one animal (see wax-model, Case 12c). 



Organ of In Cephalopods the ear, like the eye, is known to make its first 

 earing. appearance in the form of an open pit, the mouth of which 

 gradually closes up, leaving only a narrow slit in communication 

 with the exterior. It is probable that in many forms the so- 

 called ear is an organ by means of which the mollusk becomes 

 acquainted with changes in the surface over which it is passing ; 

 it is often found deeply imbedded in the substance of the foot, 

 where it forms a closed vesicle. 



Sense of There is no doubt that the carnivorous Gastropoda are gifted 



with a sense of smell, and throughout the series we observe patches 

 of modified cells of the body-wall which serve either as olfactory 

 organs or as an apparatus for testing the nature of the water of 

 respiration. 



The sexes The sexes are distinct in the most highly organized Mollusca, 



andrepro- j^ are uru t e j m ^ e same individual in some of the lower forms, 



auction. , 



such as Land-Snails, the Opisthobranchiata (including the Bubble- 

 Shells, Sea-Slugs, &c), and in some Bivalves. The reproduction 

 of Mollusca is in all cases effected by means of eggs. In some 

 instances the young are actually hatched within the oviduct of the 

 parent, as in the Freshwater Snails (Paludina); and apparently in 

 most Bivalves the eggs are also retained within the valves until 

 hatched. 



The ova of many mollusks are deposited in masses enclosed 

 in capsules. Some of them are very wonderful and complicated 

 structures. Those of the Cuttles and their allies are clustered like 

 grapes, each capsule containing but a single embryo ; but in the 

 Calamaries or Squids they form a radiating mass of elongated 

 sacks, each containing from thirty to two hundred eggs, and it 



