QUEKETT MICROSCOPICAL CLUB. 295 



tained by the Challenger expedition at Kerguelen, and the adult 

 form of the curious " chessman " spicules was described in the 

 report by Mr. Ridley and Prof. Dendy. Recently Prof. Dendy, 

 when examining a large number of stained sections which had been 

 preserved, found an almost perfect series of developmental stages. 

 He suggested the name discorhabds for these spicules. The 

 fully-grown discorhabd of L. bocagei is about 0*07 mm. in length or 

 height, and really bears considerable resemblance to a chessman. 



With the aid of diagrams the process of the formation of the 

 " chessman " spicules of Latrunculia was then described in detail. 

 It was pointed out that development was not identical in the two 

 species, and that certain well-marked differences were of great 

 interest from the point of view of the vibratory theory. Atten- 

 tion was called to the fact that while the spicule of L. bocagei 

 terminates with a structure resembling a crown, that of L. 

 aficalis has a slender projection at its apex ; this projection is 

 really the upper part of the protorhabd, the growth and elongation 

 of which continues after the deposition of silica has commenced 

 upon the shaft. 



The President concluded by saying that his observations seemed 

 *' to justify the statement that the development of a siliceous 

 sponge spicule is a far more complex process than is usually sup- 

 posed, and that the course of events is determined by a combina- 

 tion of physical and biological factors. The result appears to be 

 primarily of a non-adaptive character, and in many cases the form 

 of the spicule seems to remain entirely without significance from 

 the utilitarian point of view. Should they happen to be useful for 

 some particular purpose, however, the sponge is not slow to make 

 use of them, and in some unknown way arranges them in the 

 most suitable manner." A useful form might also be fostered 

 and improved under the influence of natural selection, although 

 that can hardly be supposed to be responsible for the minute 

 differences in the form of the spicules in the species under con- 

 sideration. 



" In short, I think we may conclude that the facts which I have 

 submitted afford evidence in favour of the view that the charac- 

 ters by which one species is distinguished from another are 

 usually of little or no importance to their possessor in the struggle 

 for existence, while adaptations are usually shared by many 

 different species." 



