MATRRIALS FOR A MONOGRAPH OF THE ASCONS. 503 



an acute angle, so that if the spicule were placed on a plane 

 surface with its gastral aspect downwards, the tips of the rays 

 would rest on the plane surface, while the centre of the spicule 

 would be raised up from it. 



The quadriradiate spicules consist of a basal system of three 

 rays, orientated in the sponge body exactly as are the rays of 

 the triradiate spicule, and a fourth, the gastral or apical ray, 

 which arises from the centre of the basal system, making in 

 the genera Clathrina and Ascandra equal angles with the 

 three basal rays, and projects free into the gastral cavity, 

 passing between the collar-cells. 



Not only is the basal system of the quadriradiate spicule 

 similar in all its relations to the whole of a triradiate spicule, 

 but it also develops in exactly the same manner. In the 

 species investigated by me not the least difference was ob- 

 served between the development of those triradiate spicules 

 which remained as such and those which, by addition of a 

 fourth ray, became quadriradiates. This accounts for the fact 

 that in nearly all species of Ascons the triradiate spicules and 

 the basal systems of the quadriradiates are so exactly similar, 

 at least in the case of the spicules of the general skeleton, 

 and differ, if at all, only in size. This fact is even more strik- 

 ing in the species of Leucosolenia, in which the spicular 

 systems have a bilateral form, than in the Clathrinidse. At 

 its first appearance it is not possible to predict to which class 

 the spicule is destined to belong, since neither the fourth ray 

 nor its secreting cell appears until the basal system has attained 

 a certain size. The fourth ray is, in fact, an adventitious 

 element, superadded to the basal system from a totally different 

 source. For this reason I have proposed in a former paper 

 the term triradiate system, to denote in a general way both 

 the triradiate spicules and the basal systems of the quadri- 

 radiates. 



It will therefore conduce greatly both to brevity and to sim- 

 plicity to describe first the development of the triradiate 

 systems, both of those which remain triradiate spicules and 

 those which become the basal systems of quadriradiates, and 



