60 OKHAMANDAL MARINE ZOOLOGY REPORT 



of irregular communicating blood spaces. One of these last is developed into a well 

 defined and most conspicuous channel, the anterior ventral pallial sinus (fig. 19, 

 Sn.pall.av.), which, arising close to the posterior extremity of the gills by the 

 confluence of several smaller sinuses, passes forwards and upwards, adjacent to and 

 parallel with the course of the axes of the gills, to near the base of the foot where the 

 sinus opens into the common branchio-cardiac trunk. This great sinus is accompanied 

 through the greater part of its course by a nerve trunk and by a rich black 

 pigmentation in the epithelial cells of the outer surface. It receives branch vessels at 

 intervals from the substance of the mantle and the main ones are also outlined in 

 pigment. 



It is noteworthy that the course of this pallial sinus and its main feeders, together 

 with those of the principal radial bundles of pallial muscles, are clearly impressed 

 as well-defined shallow grooves upon the inner surface of each valve. 



In the mantle fold on the right side of the body a further complexity is 

 introduced into this region by the necessity forced upon it to provide accommodation 

 for the enormously developed pyloric caecum (C.st.c.) and its huge crystalline style 

 (C.st.), which, with a narrow enveloping sheath of genital tissue, penetrates the pallial 

 tissue in a great semicircle parallel with and slightly dorsal to the median pallial sinus, 

 ending a short distance anterior and ventral to the anus. Its termination coincides 

 with the position of the free dorsal edge of the right branchial mesentery. 



As in other Lamellibranchs, each pallial lobe terminates in a thickened muscular 

 rim or margin pleated longitudinally into three folds throughout the whole length 

 (fig. 32). The two outer folds form the true pallial margin ; they project outwards in 

 the same plane as the shell, while the third or inmost fold, the velum or " veil," lies at 

 right angles to the other two and as an inwardly directed narrow shelf of tissue on the 

 inner side of the mantle. When the valves are slightly open these velar folds, with 

 their delicate digitate processes, extend inwards towards each other in the middle line 

 and thus form a highly sensitive strainer guard, ever ready to give instant warning of 

 contact with any would-be intruder. 



Of the two folds forming the true pallial edge, the outer may be called the 

 secretory fold, the inner the sensory. The former lies in contact with the growing 

 edge of the valve kept tightly adpressed thereto by a film of cuticular membrane, the 

 periostracum or epicuticula (periost.). This arises from the secretion of a layer of 

 glandular epithelial cells lining the base of the groove separating the outer from the 

 inner fold of the pallial edge. Fig. 32 shows how this structureless membrane curves 

 outwards and is reflected over the free edge of the shell, firmly attaching the secretory 

 marginal fold thereto. In this animal this function the binding or securing in place 

 of the secretory fold to the margin of the shell appears to be the sole duty of the 

 periostracum, as it appears not to persist over the general surface of the valves in adult 

 individuals, 



