232 THE president's address. 



these spicules proved to be so extraordinarily interesting that I 

 thought it might afford a suitable subject for my address to 

 you this evening.* 



A good deal has been written on the development of sponge- 

 spicules, and some of the published accounts appear to be very 

 contradictory. It would seem from the observations which I 

 am about to lay before you that this development is by no 

 means so simple an affair as is often supposed, but is in reality 

 a very complex process, the course of which is determined partly 

 by physical and partly by what may be termed biological factors. 



The two species of Latrunculia in question, L. hocagei and 

 L. apicalis, were both obtained by the Challenger Expedition at 

 Kerguelen, and the latter also off the mouth of the Rio de la 

 Plata, and were described by Mr. Ridley and myself in the Re- 

 port on the Challenger Monaxonida, published thirty years ago. 

 Only the adult form of the curious chessman spicule was, how- 

 ever, described, and I had no idea, until I came to re-examine 

 them the other day, that my preparations — consisting chiefly of 

 a large number of sections stained with borax carmine and cut 

 by the ordinary paraffin method — contained an almost perfect 

 series of developmental stages. 



Much as I deprecate the unnecessary introduction of new 

 technical terms, I must venture to suggest that in future these 

 spicules should be called " discorhabds " instead of " discasters." 

 The latter term, first used, I believe, in the Challenger Report, 

 was based upon a complete misconception as to the nature of 

 the spicule, which does not belong to the astrose series at all, 

 but is clearly shown by its development to be a linear type 

 modified by the addition of disk-like outgrowths or whorls of 

 spines. 



The fully grown discorhabd of Latrunculia hocagei (fig. 7) is 

 about 0"07 mm. in length — or height — and really bears consider- 

 able resemblance to a chessman. It consists of a straight, stout 



* A brief account of the gross features of the development of the 

 chessman spicule in Latritticulia lendenf eldi has been given by Hent- 

 Bchel (Monaxone Kieselschwammo und Hornschwamme der deutschen 

 Sudpolar-Expedition, 1901-3). The course of events in that species 

 evidently agrees in general with what occurs in L. bocagei and L. 

 apicalis. 



