SPONGES 103 



ated with equiangular triradiate systems, the latter with systems 

 which have the angles sagittal, and the curvature is then in the 

 plane of symmetry, being so directed that the tip of the gastral ray 

 points in the opposite direction to the posterior ray. All the 

 numerous variations of the gastral rays are quite independent of 

 the variations in the rays of the basal triradiate system. 



Arrangement of the Spicules in the Skeleton. The simplest types 

 of skeleton are seen in the Olynthus stage (Figs. 1 and 60, h), which 

 furnishes a natural and convenient starting-point for tracing the 

 evolution of the skeleton. However complicated the structure of 

 the adult sponges, in the Olynthus stage they differ from one 

 another, as has been said, by characters merely of specific value, 

 the arrangement and relations of the spicules being of a uniform 

 character. 



In the Olynthus the spicules form a single layer supporting and 

 protecting the thin body wall. The monaxons are placed more or 

 less tangentially with one end embedded in the tissues, and the 

 other extremity projecting freely on the exterior of the sponge ; a 

 situation which explains the difference between the two ends of 

 these spicules (Fig. 60, h). The triradiates, on the other hand, are 

 completely embedded in the body wall, and are so placed that one ray 

 of each triradiate points downwards, away from the osculum, while 

 the other two slant obliquely upwards and outwards to the right 

 and left. In this way an unpaired posterior ray is marked off from 

 two paired lateral rays ; but the distinction between them may be 

 one which is only recognisable when the spicules are in situ in the 

 sponge wall (regular triradiates, Figs. 1 and 42), or the spicule may, 

 on the other hand, exhibit a structural differentiation of the rays, 

 correlated with their position and function in the spjnge (sagittal 

 triradiates, Fig. 60, h). What has been said of the triradiates 

 applies also to the three basal rays of the quadriradiates, which 

 have an exactly similar orientation ; the fourth ray, on the other 

 hand, projects freely into the gastral cavity on the inner side of 

 the body wall, never towards the exterior. If the gastral rays are 

 curved, they always point up towards the osculum. 



From the skeleton of the Olynthus may be derived that of any 

 adult calcareous sponge by a series of adaptations to the structural 

 requirements of the various parts added during growth. 



In the Homocoela the skeleton retains in all parts of the body 

 the primitive arrangement in a single layer, seen in the Olynthus, 

 but exhibits marked differences in the two families of the sub-order. 



The family Clathrinidae is characterised by equiangular triradiate 

 systems, a type of spicule doubtless correlated with the reticular form and 

 growth of the sponges themselves (cf. p. 7 supra). Monaxons may be 

 present and some of the triradiates may develop gastral rays, but in 



