670 PROP. MINCHIN AND DR. REID ON THE [June 16, 



tinuous at this point with the sheath (fig. 8, PI. XXXYII.). 

 The terminal portions of the filaments are very slender and 

 delicate, but they soon become thicker as we pass towards the 

 centre of the spicule, and in the greater part of the shaft of each 

 ray the axial filament is a coarse structure very obvious when 

 stained. At the junction of the rays the filament widens out 

 very greatly and forms a cobweb-like ari'angement, usually of 

 triangular shape, which may be termed the central ti-iangle, 

 a,nd occupies the centime of the triradiate system (figs. 2, 5-7, 

 PL XXXV. "). By comparing difierent spicules, it is seen that the 

 structure of the central part varies. In those spicules which have 

 developed a fourth ray and become quadriradiates, each of the 

 three axial filaments of the triradiate system is continued into 

 the central ti'iangle, but usually not quite to the central point of 

 the spicule : the filament seems to break up as it were, to form 

 the triangle (figs. 6, 7). On the other hand, in the triradiates 

 with no gastral ray, the axial filaments are continued scarcely 

 diminished to the centre and there become continuous, and the 

 central triangle is very faint (fig. 2). This arrangement, though 

 specially characteristic of the triradiates, is sometimes seen also 

 in the quadriradiates (fig. 5). 



The monaxons of C. contorta were found very difficult to deal 

 with on account of their huge size. In the albuminised prepara- 

 tions they are only partly covered by the albumen, hence the 

 sheath stains very deeply, just as in the triradiates when they are 

 stained without being imbedded. Further, when they are cleared 

 and mounted in Canada balsam after staining, the larger mon- 

 axons collapse. Smaller monaxons, however, give satisfactory 

 preparations from which good photographs can be taken (figs. 9, 

 10, PI. XXXV.). It is seen that the monaxons contain an 

 axial filament which commences at each extremity of the spicule 

 as a fine thread, and as it passes towards the middle point of 

 the spicule the thread widens out so as to be represented by a 

 double contoured band, which extends through the greater part 

 of the shaft of the monaxon. In some monaxons the band may 

 be qviite one-third the width of the spicule ; in others it is com- 

 paratively narrower. 



The monaxons of C. contoi-ta require decalcification for not 

 less than half-an-hour, or even longer. 



The /Spicules of other Calcareous Sponges. 



In the ClathrinidBe examined, namely C. clathrus and Ascandra 

 falcata^ we have found the filaments exceedingly distinct, as was 

 noted by Minchin (1898), in G. coriacea, when stained by the 

 methods above described ; and there is scarcely any difference to 

 be noted except in minor points, from what has been described 

 in Clathrina contorta. Ascaiulra falcata is a very favourable 

 object for studying the filaments, especially in the characteristic 

 sickle-shaped monaxons (fig. 12, PI, XXXIV., and figs. 13, 14, 



