560 DR J. STUART THOMSON ON 



The colonies are of a light chocolate colour, the lighter shade being produced by the 

 presence of numerous colourless spicules. These spicules are conspicuously present on 

 the surface of both the trunk and the polyp-bearing part, but more especially on the 

 latter. The surface of the trunk is marked by longitudinal and transverse lines and 

 grooves. In the better expanded specimens, the trunk is seen gradually to taper 

 towards the base. In some cases, organic objects such as corals are seen at the base 

 of the colony. 



The extremely large number of slightly projecting spicules is noteworthy, and these 

 are so abundant as to give to the surface of the colony a certain degree of roughness. 

 For example, when a needle is drawn along the surface, a sound is produced similar to 

 that which would be heard on scraping a rough pebble. 



The polyps appear to have no definite arrangement, but are irregularly scattered 

 over the surface. They are completely retractile within the coenenchyma, and in this 

 state must be efficiently protected by the numerous spicules which surround the open- 

 ings. In my specimens the polyps are, as a rule, not well expanded, but a crown with 

 tentacles, a stomodaeal part, and a slightly developed eight-partite calyx is recognisable. 

 One of the better expanded polyps measured 2 mm. in length by 1 mm. in breadth. 

 The appearance of a partially retracted polyp is that of a slightly raised area on the 

 surface of the coenenchyma. This area is covered over, except at the centre where the 

 tentacles appear, by eight triangular parts with spicules and constituting the calyx. 



Transverse sections through the trunk show large canals internally, and smaller 

 canals externally. Numerous spicules are present in the rind. The spicules are here 

 shorter than in the external coenenchyma of the upper part of the colony ; there are 

 comparatively few in the centre of the trunk. 



PL III. fig. 18 represents a transverse section through the apical part of the 

 coenenchyma, after removal of the spicules. It shows externally numerous small 

 cavities in which the spicules were embedded, and more internally the polyps with 

 intervening capillary tubes (see footnote, p. 561). 



There is much uniformity in the shape of the spicules from different parts of the 

 colony. They are usually clubs, a few spindles also occurring. The absence of spicules 

 from the polyp (excepting calyx) is noteworthy. 



In the fully retracted condition, the calyx appears simply as a flat, eight-rayed star. 

 The diameter of partially retracted polyps varies from about 0'50 to 2 '80 mm. 



Transverse sections through the polyps show that the stomodseal wall is very 

 thick, the siphonoglyphe is ciliated, and the muscles are very feebly developed. The 

 siphonoglyphe appears to be ciliated only for a short distance. There is a strong 

 tendency to a bilateral arrangement ; the dorsal and ventral intermesenterial cavities 

 are smaller and more regular than the other six. 



The coenenchyma of the polyp-bearing part has the following structure : The 

 external part has an immense number of spicules, mostly club-shaped and directed 

 radially ; in the central part there are comparatively few spicules. The polyps are 



