DESCRIPTION OF THE PLATES. 



like a ligament in function. But the histological difference between 

 the parts does not extend to other Lamellibranchiata. 



e. The left auricle, and 



/. The single ventricle of the heart. The auricles are fused together in 

 the middle, while their two extremities are free, i. e. they receive blood 

 from the gills by two channels and communicate with the ventricle 

 by two short vessels, the ventricular orifices of which are guarded by 

 two valves apiece. The ventricle is not perforated by the intestine, a 

 point in which the Oyster resembles Anomia and Teredo. The heart 

 lies in a pericardium situated anteriorly to the adductor and com- 

 municating with the exterior through the nephridium. 



f. The body of the animal which contains the digestive tract, the liver or 



hepato-pancreas, the organs of generation, and a part of the renal 

 organ. A short ' oral process ' of Hoek, which contains a loop of 

 intestine (Fig. 2, c.}, projects from it just in front of the visceral gan- 

 glion,^. 



g. The left visceral ganglion, of very large size, and when seen from the 



surface instead of sideways, bilaterally symmetrical. It gives off 

 posteriorly and laterally nerves which branch repeatedly, possess a 

 muscular sheath (Hoek), and are connected at the margin with the 

 pallial nerve. This nerve is complete, i. e. extends along the whole 

 edge of the mantle. Anteriorly the ganglion gives off in addition 

 to the cerebro-pleural commissure two nerves, one of which passes 

 between the adductor and the pericardium to the mantle. The other 

 is the branchial nerve, and is seen passing along the commissure of the 

 two left gills. Between this nerve and the cerebro-pleural commissure, 

 and behind the spot where they cross one another, is a depression, 

 the common vestibule of the renal and generative ducts. 



h. Cerebro-pleural ganglion of the left side sending forwards branches to 

 the pallial nerve. There is no foot and in consequence no pair of pedal 

 ganglia. These ganglia are either fused with the cerebro-pleural or 

 else aborted ; but there is a nerve-cord passing beneath the mouth from 

 one to the other cerebro-pleural ganglion which represents the cord 

 connecting the two pedal ganglia of Anomia. In this animal they are 

 approximated to the cerebro-pleural ganglia and are of large size. 



i. Commissure between the cerebro-pleural and visceral ganglia. Two 

 nerves going to the body originate from it. 



j. Large funnel-shaped anus. 



The organs of generation as described by Hoek extend over the surface of the 

 body and the anterior surface of the pericardium. There are two glands, a right 

 and left, but they are connected peripherally. They form a system of anastomosing 

 and interlacing channels beneath the integument from which caeca descend vertically 



