TYPE MOLLUSC A. 333 



148, B, s), whence the greater portion passes into the compli- 

 cated network of the nephridia and thence to a blood-vessel, 

 the branchial artery, running along the base of the gill of each 

 side. After traversing the gill-filaments it becomes arterial 

 and is returned to the branchial veins which run parallel to 

 the branchial arteries and thence is returned to the auricles 

 of the heart. 



The digestive tract has a much simpler structure than in 

 the majority of the Mollusca, lacking all trace of a radula 

 and muscular pharynx. On each side of the mouth are two 

 usually triangular plates, the so-called labial palps, the upper- 

 most of which meet above the mouth forming a sort of upper 

 lip, while the lower ones similarly form a lower lip. At the 

 bottom of the space separating the two palps of each side is 

 a groove which, starting at the sides of the mouth, runs back- 

 wards along the sides of the body to the gills. This groove 

 serves for the conduction to the mouth of the particles of food 

 brought into the mantle-cavity by the action of the cilia of 

 the gills, the food of the Pelecypods consisting of diatoms 

 and other minute organisms capable of being captured in this 

 manner. The oesophagus opens into a stomach (Fig. 149, s) 

 which receives by numerous openings the secretion of the 

 usually voluminous digestive gland (7), the so-called liver, and 

 passes posteriorly into the intestine ({), which, usually in sev- 

 eral convolutions, lies imbedded in the tissues of the base of 

 the foot. In the Avail of the anterior portion of the intestine 

 is a groove, frequently converted into a canal, which may 

 open into the stomach by an independent opening ; the epi- 

 thelium of this groove or canal secretes a substance which 

 forms a transparent glass-like rod lying in the canal and pro- 

 jectiug into the lumen of the intestine. The function of this 

 crystalline style, as it is termed, has been the subject of much 

 speculation, the most plausible theory being that the secre- 

 tion serves to surround sharp-edged particles of sand or simi- 

 lar substances, taken into the intestine with food, with a jelly- 

 like coating which will prevent them from injuring the delicate 

 walls of the intestine. Towards its posterior end the intes- 

 tine bends upwards, i.e. dorsally, to a point in front of the heart 

 and then passes directly backwards to terminate in the anus 



