172 CEYLON PEARL OYSTER REPORT. 



a chestnut-brown. In one specimen this colour was very marked around the edge oi 

 the oval shield and extended a little way down the body, further on one side than on 

 the other, and then gradually faded out to reappear in a slighter degree on the 

 posterior shield. The same specimen also showed a few irregularly placed and ill- 

 defined blotches of chestnut colour near the centre of the body. 



The proboscis was not fully extended in either specimen. It was protruded from 

 the centre of the oral shield and extended in one specimen some 3 millims., in the 

 other somewhat less. The proboscis bears anteriorly several rows of chitinized 

 processes obviously homologous with the hooks of other forms. Further back it 

 bears prominent rounded papillae, which are continuous with those on the anterior 

 shield (fig. 5). This shield, like the posterior one, is separated from the sides of the 

 body by a well marked rim. The papillae on the anterior shield are uniformly 

 scattered, but on the posterior shield they run in radiating lines from the rim to the 

 thickened and somewhat indented centre. The walls of the body are comparatively 

 smooth, with at best a few low papillae, and these mostly at the two ends. 



I opened one specimen with a longitiidinal incision, and the following is an account 

 of the arrangement of the internal organs. Unfortunately the specimen had had its 

 alimentary canal broken, and the contents, consisting of sand and fragments of shell, 

 were all over the place, and much impeded observation. 



The longitudinal muscles of the skin are continuous, as in some of the species of 

 A*pidosiplion, and the interior of that covering presents a smooth glistening gray 

 surface. The alimentary canal seemed slightly coiled, but it was impossible to 

 determine if there was a spindle muscle or not. 



There are four retractor muscles, two ventral and two dorsal ; only two, and these 

 are often fused, occur in Aspidosiphon (fig. 8). The ventral are far stouter than the 

 dorsal, and arise further back at about the level of the junction of the anterior quarter 

 with the posterior three-quarters of the animal. The dorsal muscles are hardly half so 

 thick as the ventral. They have their origin a little to right and left of the anus 

 and at about the same level, which is some 2 or 3 millims. behind the edge of the 

 oral shield. 



At the base of the attachment of the large ventral muscles is a well marked fringe 

 which obviously gives rise to the reproductive cells. 



A single pair of nephridia lie one on each side of the anus. Their external openings 

 lie close to the edge of the dorsal shield, and probably just behind it. 



The introvert of both specimens was half everted, and I cut sections of one of them. 

 Unfortunately there was a little sand in the interior of the introvert, and some of the 

 sections were much broken. I was, however, able to make out that the number oi 

 tentacles is somewhere about twelve to fifteen. Each tentacle is triangular in section, 

 with a well marked ciliated groove continued down into one of the grooves which line 

 the beginning of the oesophagus. The transverse section also shows the three spaces 

 continuous with the body cavity which communicate at the tip, and which by 



