ANATOMY OF THE PEARL OYSTER. 45 



great, no two being quite alike, but most of them are doubly tritid, the summit of the 

 papilla being first divided into 3 stout branches, each of which again divides into 

 3 digitate twigs; occasionally a twin process is seen (fig. 7). The extremities of the 

 (wigs bear minute (? sensory) processes formed of groups of epithelial cells. 



In a healthy expanded oyster, where the papillate edges of the velum meet, these 

 large papillae interdigitate (Plate III., fig. 10). The short papilla?, which are placed 

 between, are simpler, and digitate in form. 



Along the posterior edge of the body, i.e., from the pallial fold upwards to the 

 posterior end of the hinge, the larger papillae of the veil become greatly reduced in 

 size and simpler in form until, in the region of the posterior "auricle" of the shell, 

 they approach in character those of the pallial margin. The same reduction in size 

 is also seen in the velar processes within the anterior auricle. 



The pallial margin consists of a conspicuously pigmented papillate free inner edge 

 and an outer fold, which is continuous with, and is covered by, the film of periostracmn 

 which folds over the free edge of the valve (Plate VIII., fig. 2, Mg.Pall. and 

 Per.Ostr.). The whole of the free margin is drawn out into very delicate and very 

 sensitive elongate digitate processes of two sizes, long and short (Plate III., fig. 8). 

 In young specimens, 1 year old, the tips of the longer papilla? bear asymmetrically 

 disposed secondary projections, sometimes simple, sometimes very abbreviated. In 

 older individuals 2 years old and upwards these longer papilla? become further 

 elongated and conspicuously fimbriated (Plate III., tig. 9). They can be seen, when 

 alive, swaying and bending gracefully about as if the tip were a tactile organ on 

 watch, feeling first in one direction, then in another. These fingerdike processes are 

 especially well developed in the region of the exhalent aperture some, a full ^ inch 

 in length, projecting even beyond the velar papilla?, which latter are here turned 

 outwards in -the same plane as the mantle lobe. The long processes stand singly, 

 separated from one another by from 2 to 1 1 short and usually simple digitate papilla? 

 closely set (Plate III, fig. 8). The appearance of the surface of a papilla in section is 

 shown in Plate VIII., fig. 2, at Mg.Pall 



The Ciliated Pallial Path. The whole of the inner surface of the pallial lobes is 

 ciliated, hut, at the ventral truncate edge of the labial palps, a specially marked 

 ciliated path begins which, curving at once outwards and downwards, passes to the 

 base of the velum, parallel with which it runs until it reaches the anterior wall of the 

 pallial fold, where it passes over the velar edge by means of a slight folding of the 

 latter (Pall./., Plate VI, fig. 14). The cilia of this pathway are in continuous action 

 from before backwards, by which means the unsuitable particles collected by the 

 gills and sent forward to and rejected by the palps are conveyed away and passed out 

 from this pallial fold (Plate VI., figs. 1, 15, &c, Pall.f.). 



When coming under the influence of the strong excurrent flow from the gills, the 

 smaller particles are frequently propelled to a considerable distance from the oyster 

 a provision to ensure that they do not again become a source of annoyance and loss 



