SHELL GALLERY. 



Sense of 

 smell. 



The sexes 

 and repro- 

 duction. 



exterior. It is probable that in many forms the so-called ear is an 

 organ by means of which the mollusc becomes acquainted with 

 changes in the surface over which it is passing ; it is generally found 

 deeply imbedded in the substance of the foot, where it forms a 

 closed vesicle. 



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 (the ospliradmm) which serve 

 either as olfactory organs or as an apparatus for testing the nature 

 of the water of respiration. 



The sexes are distinct in the most highly organized Mollusca, 

 but are united in the same individual in some of the lower forms, 

 such as Land-Snails, the Opisthotrancliia (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 {Vivipara) ; and apparently in many Bivalves 

 the eggs are also retained within the valves until hatched. 



The ova of many molluscs 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 

 has been estimated that one of the spawn-clusters of the Common 

 Squid {Loligo vulgaris) contains as many as 40,000 ova. Everybody 

 knows the spawn-cases of the Common Whelk, found so abundantly 

 on the sea-beach, consisting of a large number of yellowish capsules, 

 heaped one upon another and forming an irregularly founded mass. 

 As many as five or six hundred capsules may be piled together in 

 a single heap, each capsule containing several hundred eggs, of 

 which perhaps only thirty or forty are hatched. 



In other genera, as Tethys, Doris, Eolis, &c., the eggs are 

 contained in a spirally rolled ribbon or strap-like structure ; and 

 some of the Naticcc build a somewhat similar capsule, composed of 

 the eggs cemented together by sand and a gelatinous material, the 

 whole forming two-thirds of a circle narrowed at the upper part. 



Terrestrial Molluscs deposit, in comparison with their marine 

 relations, but very few eggs. They are sometimes covered by a 

 thin soft skin, but in certain groups, such as the large South- 

 American Strophochili and the African Achatmce, which include the 



