230 CEYLON PEAEL OYSTER REPORT. 



The E c t o d e r m (fig. 23) of the body-wall is relatively much thicker than in 

 Heteroeyathus, and is richly supplied with gland cells and nematocysts. The 

 character of the epithelial cells is well shown in the figure. The gland cells are of 

 three kinds: (l) Goblet cells with a central nucleus and filiform internal ends, the 

 wider external end filled with small granules staining deeply in hematoxylin or 

 picro-indigo-carmine. (2) Flask-shaped cells with broad internal ends and a narrow 

 neck opening at the surface ; these cells are filled with coarse granules which stain in 

 the same way as those of the first variety. (3) A few large vacuolated sac-shaped 

 cells whose contents stain with eosin, but remain colourless in picro-indigo-carmine. 



The nematocysts are of two kinds. The small spiral-thread variety is fairly 

 abundant, and here and thei*e are large torpedo-shaped nematocysts with barbed 

 threads, similar to those described for Heteroeyathus. 



The ectoderm lining the small canals leading into the Aspidosiphon chamber is not 

 modified as in Heteroeyathus, but contrariwise, it loses its glandular character and 

 consists almost exclusively of ciliated columnar cells, among which nematocysts of the 

 larger variety with barbed threads are to be found. In some of my sections the 

 lumina of the tubes are packed with everted nematocysts of this kind, which have 

 evidently been discharged when the polyp was killed, and this suggests that these 

 canals serve in some manner as special batteries of nematocysts protecting the 

 commensal Sipunculid. In fig. 25 I have given a representation of the manner in 

 which these chamber canals pass through the general mass of ccenenchymal canals into 

 the Aspidosiphon chamber. It will be observed that the endoderm canals, generally 

 twelve or thirteen in number, surrounding them are specially related to the chamber 

 canals and lie parallel to them. 



The tentacular ectoderm is raised into large and broad nematocyst batteries 

 containing numerous small spiral nematocysts and a lesser number of those of the 

 larger type. In the peristomial region the ectoderm becomes thinner, less glandular, 

 and contains but few nematocysts. At the lips it again becomes thick, and in this 

 region the nervous layer is thicker than elsewhere, and I was able to observe a few 

 large pale nuclei embedded in the layer of nerve fibrils which appear to belong to 

 gland cells. In this region, as in Heteroeyathus, the radially disposed muscular fibres 

 of the ectoderm cells are very conspicuous, and one can equally well recognise the 

 circular layer of endodermic fibres forming a sphincter oris. 



The ectoderm of the lips passes without any abrupt change into the stomodaeum. 

 Here the same elements may be recognised, but in different proportions. The 

 epithelial cells are elongated filiform, and their deeply staining elongated nuclei are 

 closely crowded together. Nematocysts of the spiral and barbed types are fairly 

 abundant, especially the latter variety. The glandular cells are much increased in 

 number, especially the eosinophile cells of the third type. The layer of nerve fibrils 

 at the bases of the cells is relatively thick. This structure is continued without much 

 change into the broad filaments of the primary and secondary mesenteries, but in the 



