42 Proceedings of the Royal Physical Society. 



and their lines of junction are almost obliterated. Fig. 6, 

 PL II., shows a series of spicules from the growing point of 

 the stolon of the red variety. Here, in addition to forms 

 similar to these described above, there are also to be frequently- 

 seen rosettes, formed by the union, more or less regularly, of 

 six wedges. 



In Fig. 5 on PL 11. a mass of spicules from the meso- 

 derm of the body wall is shown. They are drawn in 

 their relative positions, and form as may be seen a dense 

 layer. In the lower part of the body wall they are in some 

 places several layers thick. In most of them the lines 

 indicating the union of wedges have become obliterated, but 

 here and there they can still be traced, and all the various 

 forms found may be reduced to a small number of simple 

 spicules united by their narrow ends, and usually more or 

 less spinose on their surfaces. When more highly magnified 

 these spicules are found to present a minutely dotted or 

 granulated appearance, such as is represented in Fig. 4, 

 PL IL 



In the upper part of the body wall, the spicules be- 

 come slighter, less numerous, and paler in colour, and 

 the red colour of the body wall ceases altogether a short 

 distance below the tentacles. Above this, and in the 

 tentacles themselves, spicules are present, but they are slender 

 and perfectly colourless. No spicules are found in the 

 upper three pairs or so of pinnae and the corresponding 

 portion of the axis of the tentacle. Below this point a few 

 simple rod-like spicules make their appearance (PL IL, Fig. 

 10), and as the tentacle is traced downwards, the spicules 

 are found to increase in size and strength, to unite as shown 

 in Figs. 10 and 11, and to form figures like these described 

 from the stolon, but having the branches slighter and more 

 rod-like. One or two very pale red spicules were found in 

 the tentacles, near to the base. 



The endoderm of the body wall is a thin layer lining the 

 inner surface of the mesoderm. It is formed of irregular 

 cells mostly somewhat spherical or columnar through mutual 

 pressure, and apparently in some places several layers deep. 

 In some sections many rounded granular cells were found 



