3^ THE president's ADDRESg. 



those which are obviously adaptive in character and those which 

 seem to have no sort of relation to the requirements of the 

 organism in which they occur. This point is very well illus- 

 trated by the form and arrangement of the spicules. 



It is obvious that spicules, being secreted by the living pro- 

 toplasm of mother-cells or scleroblasts, can only make their 

 appearance in situations where such cells occur — that is to say, 

 speaking generally, the mesogloea or middle layer of the sponge- 

 wall. Hence, in the simplest calcareous sponges, of the genus 

 Leucosolenia, where the entire body consists of a single thin- 

 walled tube, we find the skeleton composed chiefly of a system 

 of triradiate spicules lying in the middle layer of the wall, where 

 their interlacing rays form a kind of lattice-work. In the inter- 

 vals of this lattice-work lie the inhalant pores or prosopyles, 

 and the form of the triradiate spicule is obviously related 

 to the arrangement of these apertures in the wall of the tube, 

 the rays being extended in the intervals between the pores 

 and, at the same time, where necessary, becoming curved so 

 as to follow the curvature of the wall of the tube. The ar- 

 rangement of the skeleton as a whole, then, and the form of 

 the principal spicules alike appear to be determined in the 

 first instance by the arrangement of the canal-system and to 

 be due to easily understood mechanical causes. Even in such 

 simple forms as Leucosolenia, however, we usually find two 

 further adaptive modifications of the skeleton. Some of the 

 triradiate spicules may develop a fourth or " apical " ray, 

 which springs from the centre of the spicule and projects 

 into the cavity of the tube (gastral cavity), where it serves as 

 a protection against internal parasites. A corresponding protec- 

 tion of the external surface is effected in a different manner, by 

 the development of a monaxon spicule pointed at both ends, 

 arranged so that one end projects obliquely outwards and up- 

 wards, giving a prickly or sometimes hairy character to the 

 surface. The relations of the monaxon form to the triradiate are 

 somewhat obscure, but we know from the researches of our late 

 President, Professor Minchin, that the triradiate of the calcareous 

 sponges is not a simple spicule but a spicule-system, formed 

 by the union of three monaxons each initiated by a separate 



