224 CEYLON PEARL OYSTER REPORT. 



but it is remarkably modified in tbe inner moieties of tbese tubes. It becomes much 

 thicker (Plate III., fig. 17) and has a vacuolated appearance, and in place of the 

 gland cells described above we find elongated pyriform gland cells filled with round 

 granules, which stain deeply in hematoxylin. But the most striking feature is 

 afforded by the nematocysts. These are very numerous, closely crowded together, 

 are ovoid in shape and very large, with a thick thread covered with barbs arranged 

 in a double spiral. They contain coarse, closely-packed granules, which stain 

 intensely blue in picro-indigo-carmine. In spite of their much larger size (figs. 16 

 and 17 are drawn to the same scale) and their different shape, I regard these as 

 modifications of the ordinary nematocysts of the ectoderm of the body-wall. But 

 they are certainly very strikingly modified, and I am at a loss to explain their 

 function, situated as they are at the deeper end of tubes whose function is also 

 problematical. 



The ectoderm of the tentacles is raised into a great number of the well-known 

 knob-like " batteries," crowded with small nematocysts of the usual form with a 

 closely coiled spiral thread. Interspersed among these are larger nematocysts 

 resembling those of the body-wall, but usually much longer and narrower. The 

 ectoderm of the peristome is extremely thin, and consists of a cubical epithelium with 

 very few gland cells, and, as far as I could ascertain, no nematocysts. At the lips of 

 the mouth, however, the ectoderm is thickened, and shows some special features not 

 visible in the ectoderm of the body-wall. There are very few gland cells, those 

 which are present containing coarse granules staining blue in picro-indigo-carmine. 

 I could not detect any nematocysts. Muscle fibres, which were scarcely distinguishable 

 in the peristome, are here well developed and arranged radially so as to form a more 

 or less distinct dilator oris muscle. In many sections the epithelio-muscular character 

 of the ectoderm cells becomes evident. The layer of nerve fibres at and between the 

 bases of the ectoderm cells is distinct, and among the epithelio-muscular cells very 

 small attenuated, spindle-shaped cells may be distinguished which may be interpreted 

 as sense cells. This thickened ectoderm passes insensibly into the endoderm at the 

 lower margin of the lips, except at the places where the twelve primary mesenteries 

 reach the mouth. Here it is continued into the very thick ectoderm of the Y- or 

 T-shaped filaments of these mesenteries. In these filaments, as mentioned above, 

 the ectoderm consists almost exclusively of very long attenuated, ciliated, epithelial 

 cells, whose character is sufficiently indicated in fig. 14. There are very few 

 glandular elements and no nematocysts. The coiled filaments of the bases of the 

 primary mesenteries, and nearly the whole extent of the filaments of the secondary 

 and tertiary mesenteries, consist of closely-packed, attenuated, epithelial cells, among 

 which numerous gland cells and nematocysts are wedged in. The gland cells are of 

 two kinds : (l) ovoid vesicular cells with clear contents ; (2) pyriform cells containing 

 coarse granules staining deeply in picro-indigo-carmine or hsematoxylin. The 

 nematocysts are all of the type shown in fig. 18. They are very large, measuring 



