Vibrations upon the Form of Certain Sponge-Spicules. 575 



exactly as might be expected if it really indicates the position of a node. Of 

 course individual variations occur, due, doubtless, to various disturbing factors, 

 but these do not invalidate the general conclusion that the thickenings 

 represent nodal points. 



As already stated, the whorls of the Zatrunculia spicule are not sufficiently 

 well denned or regular on their first appearance to yield accurate measure- 

 ments of their position. There is, however, amongst the remarkable 

 collection of sponges obtained by the " Sealark " Expedition in the Indian 

 Ocean, a species belonging to a new genus related to Zatrunculia, in which a 

 discorhabd occurs which is much better suited for mathematical investiga- 

 tion. As the species has not yet been described, we may refer to this spicule 

 simply as the oxydiscorhabd, from the fact that it is an oxeote spicule with 

 two disc-like whorls. The shaft of the spicule (fig. 15) is thickest in the 

 middle, and tapers gradually to a sharp point at each end, being approxi- 

 mately symmetrical but slightly curved. The young spicule (figs. 4-14) may 

 be slightly angulated in the centre and, as is frequently the case in oxeote 

 spicules, may exhibit a slight central enlargement (the primary central 

 thickening), which, however, as will appear later on, has nothing whatever to 

 do with the formation of a whorl. When fully developed, the surface of the 

 shaft is slightly roughened with numerous small points. The spicule evidently 

 belongs to the common diactinal type, in which two of the rays of the 

 primitive tetract have been suppressed, and, before the formation of the 

 whorls (fig. 4), it is doubtful whether it could be distinguished from the 

 young oxeote megasclere of the same sponge. The whorls, when fully 

 developed, are thin discs of silica (fig. 15), not divided into lobes, but 

 with an irregular margin ; the median one is concave on the side facing the 

 other, which latter evidently corresponds to the subsidiary whorl of the 

 Zatrunculia spicule. 



Unfortunately the only specimen in which this oxydiscorhabd occurs is 

 not sufficiently well preserved to enable us to say anything about the cells 

 which are concerned in the development of the spicule, but it seems 

 reasonable to assume the existence of formative cells having the same 

 relation to the whorls as in Zatrunculia bocagei. 



Only two whorls are developed, and these do not appear until the shaft 

 has attained a considerable thickness, measuring about O066 mm. in length 

 by - 0023 mm. in diameter in the middle. Then the whorls appear, almost, 

 if not quite, simultaneously, the median one at or very near the centre of the 

 shaft, and the subsidiary one at one side of it, always nearer to the centre than 

 to the end. At first, each whorl is represented by a thin, sharply defined ring 

 of silica, the position of which can be accurately determined by measurement. 



