332 INVERTEBRATE MORPHOLOGY. 



what extensive development in connection with the presence 

 of the bivalved shell. The mantle-folds are as a rule some- 

 what richly provided with muscle-fibres especially near the 

 margin ; and where siphons are developed some of the fibres 

 are specialized into retractors for these organs. For the closure 

 of the shell- valves, however, more extensive muscular bands are 

 present which seem, like the siphoual retractors, to be special- 

 ized portions of the mantle musculature. Of these shell- 

 adductors there may be one, as in Ostrea and Pecten, or two, as in 

 Anodon (Fig. 149, cm and pa], which pass transversely across 

 the body from one shell-valve to the other, in the form of 

 stout compact muscular bands. In connection with the foot 

 special bauds are also developed which function as protrac- 

 tors (pp), retractors (rp), and elevators arranged in pairs and 

 extending from the inner surfaces of the shell-valves to spread 

 out below in the foot. These various bundles seem to cor- 

 respond to the spindle-muscle of the Gasteropods. 



The ccelom presents an arrangement similar to that of 

 other Mollusca, both schizoccelic and enteroccelic portions 

 being distinguishable. To the former portion belong the 

 numerous lacuuar spaces which traverse the body and mantle- 

 folds, and to the latter the pericardial cavity (Fig. 149, p) and 

 the cavity of the reproductive glands. The blood-vascular 

 S} T stem consists of a heart provided with two lateral auricles 

 and lying in the pericardium. In the majority of forms the ven- 

 tricle (y) seems to be traversed by the terminal portion of the 

 digestive tract, a condition produced by its having folded itself 

 longitudinally around the rectum, and which recalls what 

 occurs in certain Diotocardiate Gasteropods (see p. 305). This 

 arrangement does not, however, obtain in all forms, some of 

 the more primitive (Nucula, Area] having the ventricle 

 entirely dorsal to the intestine, as it is in the Amphineura, for 

 example, while in a few others (Ostrea) it has assumed a 

 secondary position ventral to the intestine. From both the 

 anterior (ao) and posterior extremities of the ventricle arteries 

 arise which, after branching a number of times, pour the blood 

 into the schizoccelic lacunar system. Traversing this the 

 venous blood is returned to a longitudinal sinus lying in the 

 middle line of the body just below the pericardium (Fig. 



