112 A Guide to the Zoological Collections in the 



Case 179, in which there can be seen (l) the alimentary 

 canal coiled and embedded in a dark digestive gland or 

 " liver", and ending in the fore-part of the mantle-chamber 

 on the right side ; (2) the two-chambered heart into which 

 the blood-vessels that branch' over the breathing-organ 

 are collected ; (3) the excretory organ or kidney running 

 alongside the great blood-vessel that proceeds to the 

 heart; (4) the coiled reproductive glands with their ducts 

 and accessory glands (in one case a part of the duct is 

 full of eggs); (5) the nerve-ganglia, collected in three pairs 

 in the vicinity of the front part of the alimentary canal^ 

 with the nerve-cords that connect the ganglia. There 

 should also be noticed opening into the front part of the 

 alimentary canal, (6) the arborescent salivary glands with 

 their long ducts, and (7) the sack containing the " odonto- 

 phore '\ an organ to be presently described. All these 

 parts can be easily seen in the dissections. 



The Mollusk shell, as already stated, is formed by the 



calcification of the superficial layer of the mantle, and 



therefore increases in size as the mantle grows. The 



variations in the shape of the shell will be afterwards 



considered. 



The Molluscan "foot" is a characteristic organ of 

 the highest importance : it is often divided by constric- 

 tions into three distinct parts, — a front, a middle, and a 

 hind part, known respectively as "propodium", " meso- 

 podium", and " metapodium ", which may undergo various 

 modifications and suppressions. 



Another characteristic Molluscan organ is the "tongue '*, 

 or " odontophore ", or "radula". It is a horny strap, 

 the upper surface of which is generally covered with 

 small sharp teeth arranged in rows like a rasp : in use 

 it is worked backwards and forwards by special muscles, 

 and in repose it lies coiled up in a sheath at the floor of 

 the mouth. The typical odontophore is well shown in the 

 dissected specimen of the Limpet above Case 128, in which 

 it is of enormous length. 



