62 OKHAMANDAL MARINE ZOOLOGY REPORT 



pearl oysters {M. vulgaris) found living in shallows — say under one fathom in depth — 

 are usually much more brightly coloured with black and orange over the mantle, gills 

 and foot than those from depths of seven to eight fathoms. We also know the general 

 tendency of continued exposure to bright light to encourage the development of 

 pigments, usually dark in hue. Hence, as P. placenta is essentially an inhabitant of 

 shallow waters, where light is intense and long-continued, we can readily understand 

 the cause of extreme pigmentation of the mantle. As to the less obvious reason for it 

 being on the exterior of the mantle instead of the interior, as' in M. vulgaris, this is 

 most probably due to the transparency of the valves. In the true pearl oysters the 

 valves are opaque — the sun's actinic rays cannot penetrate their substance — ^hence 

 no protection need be given from this aspect to the delicate organs and tissues, 

 nerves especially, lying beneath. Protection has, however, to be given against light 

 entering between the open edges of the shell, hence development of pigment on the 

 gills and the inner surface of the mantle. 



In Placuna the valves are transparent, and, as light passes through them without 

 difficulty, protection must be provided against light impinging directly on the mantle 

 from the exterior — effected, as we have seen, by a development of black pigment 

 either generally over the exterior of the mantle or restricted to the course of the main 

 trunks of the pallial nerve plexus. 



THE MUSCULATURE. 



Like the pearl oyster, P. placenta is monomyary, a single adductor, the posterior, 

 being present. The other muscles are relatively weak, and consist of : — an unpaired 

 weak levator of the foot, a similarly unpaired weak pedal retractor, an imperfect 

 orbicular pallial muscle, branchial cords, and the heart or cardiac muscle. (The last will 

 be treated of separately when describing the vascular system. ) 



The Adductor Muscle of the shell is situated sub-centrally, stretching trans- 

 versely from valve to valve midway between the anterior and posterior limits of the 

 shell, but slightly nearer the dorsal than the ventral margin (fig. 4, Add.). It is the 

 only large muscular mass in the body, and is by far the most important. The outline 

 in section is circular, while the diameter in specimens of 14 cms. long is about 2 "3 cms. ; 

 the length is inconsiderable in consequence of the extreme lateral compression of the 

 valves. It lies ventral to the visceral mass ; the surface of the dorsal half is closely 

 embraced by the renal organ. Along the posterior border runs the terminal section of 

 the rectum, while upon the antero-ventral curvature rests the parieto-splanchnic 

 ganglionic mass. 



Even a casual observation in the living condition reveals the fact that this muscle 

 is not homogeneous ; in preserved specimens the fact is emphasized ; two distinct 

 regions are visible to the naked eye — a posterior and median, and an anterior and 



