250 CEYLON PEARL OYSTER REPORT. 



they are much more highly developed than in Jousseaumia, but there is a corre- 

 spondence between the thinner epithelium lining the lateral pouches in his figure and 

 in mine which leads me to believe that we have here an indication, though in a very 

 much reduced form, of these ancestral structures. 



The oesophagus is triangular in section and lined by a richly ciliated columnar 

 epithelium. It passes insensibly into the capacious stomach, whose anterior walls are 

 richly ciliated (fig. 13, st.) t but posteriorly the lining epithelium changes in character. 

 Laterally and ventrally the cells retain their columnar epithelial character, but 

 dorsally (figs. 14 and 15, yl.c.) they lose their cilia and become glandular. The cells 

 throughout this region are rather long and columnar, and are full of green refringent 

 granules. It is in this region that the thick cuticular lining of the stomach begins, 

 and I have little doubt that these glandular cells of the dorsal wall secrete the 

 cuticle, and give rise to the crystalline style with which the cuticle is continuous. 

 The liver lobes are four in number, a right and left dorsal and a right and left 

 ventral. They open into the stomach near its posterior end, just in front of the 

 commencement of the intestine and caecum, by wide ducts on either side, the ducts 

 of the dorsal and ventral lobes of each side uniting just before they open into the 

 stomach. The liver cells were too much macerated to enable me to say anything 

 definite about their histological characters. The left upper end of the stomach is 

 prolonged into a large conical caecum (figs. 1, 16, and 17, cce.), which projects 

 backwards into the posterior part of the visceral mass and is a conspicuous object in 

 specimens mounted whole. The caecum is lined throughout by a very definite cubical 

 epithelium, whose cells bear short, stiff, bristle-like cilia, as is the case in the caeca of 

 other Lamellibranchia. In the anterior part of the caecum the cells of its dorsal wall 

 are transitional between the ordinary caecal cells and those of the dorsal wall of the 

 stomach, for they are filled with the green refringent granules, while retaining their 

 cubical character and their stiff brush-like cilia. The caecum is separated from the 

 stomach by a constriction, and at the constriction the epithelial cells are elongated 

 and their ends are produced into rather long irregular processes, apparently formed 

 of fused cilia. These processes seem to form a straining apparatus, preventing 

 particles of any size from entering the caecum, for while the stomach, intestine, and 

 rectum are full of the skeletons of diatoms, the caecum is always devoid of such 

 contents. The crystalline style is very large in some specimens, but small, or even 

 wholly absent, in others. It projects some way forward into the stomach and 

 some way back into the caecum, but seldom extends to the posterior end of the 

 latter. 



The intestine leaves the stomach on the right lower side, close to the opening of 

 the Ciucum. It runs backwards as a widish, thin-walled ciliated tube as far as the 

 posterior end of the caecum, where it turns upwards and forwards to reach the dorsal 

 surface of the visceral mass ; there its diameter narrows to form the rectum, and it 

 Ixmds sharply backwards, running parallel with the posterior margin of the shell 



