50 Records of the Indian Museum. [Voi,. XIX, 



retractor muscles are developed in some genera along the axis of 

 the gills. They serve to throw the gills into folds when the 

 siphons are retracted and withdrawn between the valves. 



The gills are of synoptorhabdic type, i.e. the filaments are con- 

 nected by interfilamentar tissues. They are either simple and 

 homorhabdic, or plicate {i.e. thrown into vertical folds) and 

 heterorhabdic (sometimes very indistinctly). 



Alimentary Canal, The oesophagus may vary in its course. 

 The stomach has been divided by Bloomer (5) into four portions. 

 The antero-ventral {oesophageal) is continuous with the oesophagus 

 in front and separated from the antero-dorsal portion {cardiac) by 

 a muscular ridge. The middle portion {central) is ventro-lateral in 

 position adjoining the oesophageal portion. The posterior portion 

 {pyloric) is separated from the central portion by a ridge passing 

 ventrally, and from the cardiac portion by another passing to the 

 dorsal aspect. 



The pyloric stomach gives off the coccum of the crystalline 

 style either from its ventral aspect or from its posterior end. The 

 course of the coecum is somewhat different in the different genera. 



The origin and course of the intestine may be thus considered 

 in a tabular form : — 



I. Solen type. The intestine arises directly from the ventral 

 aspect of the pyloric chamber independently of the coecum. It 

 consists of two limbs which pass along the dorsal and ventral 

 aspects of the coecum respectively. The type may show the 

 following variations : — 



{a) The two limbs of the intestine simple (not folded) {Cera- 



tisolen^ Subcullellus). 

 {b) The anterior limb forming a number of large folds 



{Pharella). 

 (c) The anterior limb with closely placed coils and posterior 



limb simple {S Hi qua), 

 {d) The anterior limb with closely placed coils and posterior 



limb with one or more folds {Solen). 

 (e) The anterior limb with closely lying coils and posterior 



limb with a number of large loops {Ensis). 



II. Novaculina type. The intestine arises directly from the 

 pyloric stomach and forms several long loops traversing the entire 

 length of the visceral mass. 



III. Solecurtus type. The intestine arises in connection with 

 the coecum as a common tube and is only separated from it out- 

 wardly by two lateral grooves. It forms a separate tube near the 

 distal end of the coecum and then passes along the ventral aspect 

 of the coecum (corresponding to the second limb of the first type). 

 The course is either simple or is attended with loose or close coils. 

 This type of intestine is also found in members of the fam. 

 Psammobiidae to which some of the genera of the Solenidae are 

 undoubtedly closely related. This point has already been empha- 

 sized by Dall (23) and Bloomer (16). 



