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POPULAR CONCHOLOGY. 



the side one or two openings, or even tubes; one is the 

 anal aperture, the under one the breathing tube. The 

 presence of such tubes is indicated by a sinus impressed by 

 the mantle upon the inner surface of the valves ; the larger 

 this sinus is, the longer have these tubes been. 



The main mass of the nervous system consists of three 

 pairs of ganglia ; these are connected by cords of commu- 

 nication, and send off nerves to each organ of the body. 

 The tentacles, which stud the edges of the mantle, fulfil 

 the office of feelers, as do also the before-mentioned mouth 

 feelers. The organs of hearing are simply two round 

 capsules, filled with a clear humour, and containing a 

 polished bullet-shaped body formed of carbonate of lime. 



Eyes are found in many conchifera ; these are always 

 situated on the edge of the mantle, or on the outer side of 

 the mantle tubes, differing in number, and in being pedun- 

 culated or not ; they are very completely organised. The 

 most striking are the beautiful emerald eyes of the Pecten, 

 situated between the tentacles.* 



These Molluscs live in water, both salt and fresh, 

 and occur in every zone. Some live altogether concealed 

 from sight in sand and mud, and they only betray 

 their locality by a little hollow, out of which they now 

 and then spirt up the water. Some are completely 

 visible, fixed to one spot, as oysters, or attached by 

 their byssus, as mussels. Some lie ensconced in holes of 

 rocks, corals, the thickness of other shells, or in wood, and 

 many of them possess a remarkable aptitude in boring for 

 themselves such receptacles. In the Pholas and Teredo, 

 little flinty particles are imbedded in the fore-part of the 



* These eyes disappear, some writers assert, as the mollusc grows 

 older ; but there are many rudimentary organs of sight on the border 

 of the mantle. 



