220 AN INTRODUCTION TO THE STUDY OF FOSSILS 



and possessing a ventral, often hatchet-shaped, foot capable 

 of burrowing in sand. 



There is no distinct head, nearly the whole ventral aspect 

 of the animal consisting of the muscular mass of the foot. 



The presence of a siphon in the various species of pelecypods 

 is nearly always indicated on the valve by a pallial sinus (see 

 page 214), though in a few exceptional species there is no such 

 indication of its presence. When no pallial sinus is present, 

 the pallial line is called simple. The portion of the mantle 

 which is outside of the pallial line bears the pigment glands and 

 in certain pelecypods papillae and tentacular processes. On 

 this edge likewise are the visual organs when such are present, 

 as in Pecten. 



Pearls are pathological products of the secreting function of 

 the general mantle surface, not of its edge. Small foreign ob- 

 jects, usually some parasites such as the larvae of certain worms, 

 get between the mantle and the shell and set up an irritation 

 there. The secretion of pearly deposit by the irritated glands 

 around these offending objects results in pearls. (See also 

 page 215.) 



All surface irregularities of the shell, such as lines, knobs, 

 spines, etc., are simply the result of corresponding modifications 

 of the margin of the mantle. Thus spines indicate the exist- 

 ence of finger-like projections extending out from the edge of 

 the mantle and secreting around themselves these hollow shelly 

 processes. Ribs on the shell indicate wavy undulations of the 

 mantle margin. The hinge teeth are probably derived from 

 the crenulations or ribbing of the surface of the shell. 



The digestive canal is much coiled, consisting of mouth 

 oesophagus, stomach (into which open two large digestive glands) 

 and intestine, ending in an anus. The blood of pelecypods con- 

 tains nucleated amoeboid corpuscles and in some forms, such as 

 Area pexata, — the bloody clam, some species of Tellina, etc., 

 there are also present non-amoeboid corpuscles containing 

 haemoglobin ; in such cases the blood is red. In other forms 



