Structure. 9 



Tht' s/nll is secreted by the outer surface of the mantle, but 

 may be enveloped by the mantle so as to become partly or 

 altogether an internal structure. It is formed of a horny sub- 

 stance (conchiolin) impregnated with carbonate of lime. As a 

 rule two layers can be distinguished, differing in their minute 

 structure and in their mineralogical constitution. In the outer 

 or prismatic layer, which forms most of the thickness of the shell, 

 the carbonate of lime is in the form of calcite. In the inner or 

 nacreous layer (' mother-of-pearl ') it is in the harder form of 

 aragonite. An outer skin (periostracum) is frequently jDresent. 

 In some molluscs the mineral constituents are reduced or absent 

 and the shell is ' horny ' and flexible, and in some others the shell 

 is entirely absent. 



There are one or two exceptions to the rule that molluscan 

 shells are secreted by the mantle. In the Cephalopod Argonauta, 

 as will be explained later, a spiral shell is produced by secretion 



Fig. 3. 



Mandibles of Gastropoda. A. Janus ; B. dorsal mandible of Aegirus. 

 From Lankester's Treatise of Zoology (by permission of Messrs. A. & C. Black). 



from two of the arms, while the under surface of the foot secretes 

 a calcareous plate in the Gastropod Hijyponyx. The modifica- 

 tions of the shell in the various classes will be described in the 

 systematic survey (p. 19), but the visitor's attention may here 

 be called to the exhibit in Wall-case A, where the main points 

 of shell-structure and the more remarkable changes of form 

 throughout the phylum are illustrated. 



Although the shells of many families are highly characteristic, 

 there are several examples of molluscs of widely different structure 

 and systematic position that have shells resembling each other, 

 and classifications based on the shell are subject to change with 

 increasing anatomical knowledge. 



Certain molluscs have internal skeletal structures. The head 

 cartilages of the Cephalopoda and the cartilaginous supports of 

 the radula in the Gastropoda are of this nature. 



The alimentary canal is usually differentiated into a mouth, 

 pharynx, oesophagus, stomach, and intestine. The liver opens 

 by one or more ducts into the stomach, and salivary glands are 

 invariably present. The names ' liver ' and ' salivary glands ' 



