HORNELL ANATOMY OF PLACUNA 61 



The sensory fold between the velum and the secretory fold of the mantle edge is 

 very similar in general appearance to the velar edge, but is furnished with more 

 numerous simple unbranched digitate processes. These are highly contractile, and 

 extremely sensitive. 



Along the base and in the connective tissue filling of the thrice-folded pallial 

 margin runs a peripheral blood-vessel, the pallial artery ; with it, in intimate 

 association, is a strong nerve trunk, giving off a multitude of fibrils to the secretory 

 and tactile areas. The ultimate twigs of the pallial muscles penetrate to this margin, 

 enabling retraction to be effected upon any irritation experienced by the sensory cells 

 in the vicinity. The greater the rapidity of retraction the greater the immunity from 

 damage by predatory fishes and other enemies. Large spaces or sinuses are very 

 conspicuous, penetrating the loose connective tissue which is freely developed close to 

 the mantle edge. 



We have already noted that the continuous growth of the periostracum functions 

 to keep the margin of the mantle attached to the edge of the shell. The mantle is 

 further kept in position lining the valves by the adhesion occasioned by the formative 

 secretion of the cells of the external surface. Violent contraction of the pallial muscles 

 impairs these two mechanical adhesions by dragging the mantle edge away from the 

 margin of the shell, while the distension and turgesceuce of the mantle with blood 

 coincident with relaxation of the pallia! muscles permits the contracted mantle to 

 re-spread or re-expand over its normal and full area, when a short period of quietude 

 re-establishes mechanical adhesion with the shell by new periostracal growth and 

 surface secretion. 



It is important to note the importance which the right mantle lobe assumes over 

 the left in this mollusc, through the penetration aud lodgment within it of the bulk of 

 the reproductive gland, the great pyloric caecum and a considerable portion of the 

 rectum. 



PIGMENTATION. The development and distribution of pigment in the mantle is 

 very variable. In some cases practically the whole of the external surface is covered 

 with black pigment. In other instances the colouring matter is less extensively 

 developed ; bands of pigment, however, invariably follow the course of the great 

 median pallial sinus and its main branches, as well as that of the more important of 

 the pallial muscle bundles. This black pigmentation is limited to the mantle, where 

 it occurs in the superficial epithelial cells. Nowhere else is there any true 

 pigmentation the inner surfaces of the mantle and all the folds and processes of the 

 mantle edge are colourless, a condition the reverse of what is seen in the true pearl 

 oysters (Margaritifera spp.), where the mantle edge and the inner surface of the 

 mantle are often profusely pigmented and where the outer surface of the mantle lying 

 in contact with the nacre of the valves is invariably free from even the slightest 

 pigmentation. The reason is not far to seek. It has already been noted that Ceylon 



