254 Mr. W. J. Sollas on the 



tion, produced by the probably commensal parasite. The 

 crypts are lined by an epithelial membrane containing nume- 

 rous cylindro-stellate spicules. The pillars of the crypts are 

 traversed by the long-shafted spicules, and consist partly of 

 mark-substance, and partly of vacuolated connective-tissue 

 cells, which sometimes form a distinct layer beneath the epi- 

 thelium (PI. XII. fig. 27). Sometimes the mark-cells of the 

 pillars are elongated into spindle-shaped fibres, which do not 

 generally differ, except in shape, from ordinary mark-cells, but 

 sometimes become hyaline and vacuolated (PL XL fig. 15, g 

 and v) . 



4. The Incurrent Tubes. — The tubes are simple excavations 

 in the mark, lined by epithelium, which consists of a single 

 layer of flattened cells, furnished with a round nucleus and 

 nucleolus, but with indistinct or invisible cell-borders. 



5. The Excurrent Tubes. — The smaller canals (PL XII. 

 fig. 32) of the excurrent system do not differ from the corre- 

 sponding incurrent tubes in structure ; but the larger tubes 

 have walls of a much more complex character. The large 

 vessels, for instance, which open into the oscular tube are 

 first lined by an epithelial membrane containing fine fibrils 

 and round or oval nuclei with their nucleoli ; beneath this 

 follows a colourless transparent layer, which scarcely stains 

 with carmine, and attains a thickness of 0*0007 inch. It 

 consists of fine fibres (PL XL fig. 15, /) of considerable 

 length, with a swollen middle part, in which a central round 

 granule or small nucleus may sometimes be discerned, and of 

 vacuolated connective-tissue cells, which, when they lie imme- 

 diately under the epithelium, sometimes contain a sharp- 

 rayed stellate spicule. The ruga3 of these vessels consist of 

 an extension of the fine fibrillar layer covered by the epithe- 

 lium. Globate and small acerate spicules occur in the walls 

 of these vessels. 



6. The Oscule and Oscular Tube. — The wall of the oscular 

 tube below the cortex (PL X. fig. 2) is 0'02 inch thick, and 

 consists for the most part of fibrous tissue, which does not 

 stain with carmine, and is traversed by a number of small 

 acerate spicules, which project from it erectly, and thus pro- 

 duce the hispid appearance of its surface previously mentioned. 

 Vacuolated connective-tissue cells occur intermingled with 

 fibres on both the inner and outer face of the wall ; and the 

 outermost layer consists of epithelial membraue. On the 

 inside of the wall the epithelium is associated with minute 

 cylindro-stellates like those of the epidermis, on the outside 

 with larger sharp-rayed stellates like those of the mark. In 

 places the fibrous tissue of the wall passes into true muscular 



