68 CEYLON PEARL OYSTER REPORT. 



cerebral ganglion. The osphradium is an area bounded by a well-marked projection 

 close to the parieto -splanchnic ganglion at the origin of the branchial nerve. It has a 

 small ganglionic mass lying at its base (Plate IX., fig. 18), and its nerve is cerebral in 

 origin. 



There are probably sensory cells in the ciliated epithelium on the grooved and 

 corrugated oral surfaces of the labial palps (Plate IX., fig. 11); but the sense of 

 touch seems to be localized chiefly in the margin of the mantle, and more especially in 

 the filiform and digitate processes of the velum. These latter are extremely sensitive 

 to touch, and the longer processes of the anterior and of the ventral margin have 

 compound digitate apices; it is upon these multiple terminations (Plate IX., figs. 19, 

 20) that the chief sensory epithelial cells are disposed. The long processes of the 

 posterior part of the mantle edge are of a different form. They are cylindrical filiform 

 organs beset on all sides with short spinulate branches towards the tip (Plate III., 

 fig. 9). Those on the margin of the temporary exhalent aperture are notably 

 devekyped, and in life sway and sinuously bend in snake like motion unceasingly. In 

 sections they show a delicate columnar epithelium which is no doubt sensory. 



Little can be said definitely regarding the sense of sight, although we have some 

 evidence of the function being performed to a certain extent. Thus from the 

 observations described in Part I., it is clear that there is a marked sensibility to 

 light and shadow -a sensibility which may be termed dermatoptic, as it resides in 

 the surface layer of certain regions. When the tanks were well lighted during the 

 day, the shadow of a hand passing over was frequently followed by the immediate 

 closing of the oyster's valves, and conversely after dusk they showed a similar but 

 more accentuated re-action when stimulated by a bright light. Especially is this the 

 case when surprised during a promenade, when having slipped their byssal cables 

 they crawl along in search of a new resting place. At such times, or when forming 

 new byssal threads, they appear extremely sensitive ; they cease operations immediately 

 and remain passive, with valves closed, as long as the irritation is continued. This 

 photoscopic or dermatoptic sensibility can be located only in the soft parts turned 

 towards the light the edges of the mantle and the surface of the foot, when the 

 latter is protruded and there we invariably find patches of more or less deeply 

 pigmented epithelial cells. 



THE REPRODUCTIVE ORGANS. 



The sexes are separate in Margaritifera vulgaris, and, as is shown by the experiments 

 detailed on p. 125 in Part I., remain the same from season to season, i.e., each 

 individual is permanently either male or female throughout life. 



The gonads {Go., Plate VI., fig. 1) are paired but asymmetrical. The pair together 

 forms a thick envelope covering the stomach, liver and first two sections of the 

 intestine, and thus constitutes the greater part of the outside of the proximal portion 



