126 I. IJIMA : HEXACTINELLIDA. I. 



subsequently the growth of the spicules on either side of the 

 layer takes place ivithin the limits of their respective trabecular 

 systems. 



The outer set of the choanosomal spicules exhibits a much 

 greater development than the inner in the number of individual 

 spicules, some of which here attain their largest size and also 

 frequently group themselves into compact fascicles. This peculiar- 

 ity is evidently correlated with the relatively large and continuous 

 extent of, and the abundance of trabeculœ in, the space occupied 

 by the set in question. Included in the set are all the numerous 

 and long proximal rays of the dermalia and the greater part of 

 the entire parenchymalia. Amongst the latter belong here the 

 most important parts of the skeleton, viz., all the beams of the 

 skeletal framework. These lie apparently in the deepest fundus 

 of the recesses belonging to the external trabecular layer and 

 consequently close to the gastral surface of the sponge-wall. We 

 may consider the chamber-layer as properly lying closely inside the 

 framework and as forming protuberances wherever the meshes and 

 other interstices of the latter permit. Exactly the same relation 

 plainly obtains between the dictyonal framework and the chamber- 

 layer in the Dictyonina. In Eupledella it is not always easy to 

 clearly make out the relation on sections, owing to the confusing 

 intermixture of spicules and chambers ; however, the appearance of 

 the numerous small and shallow excurrent canals occurring all 

 over the gastral surface of the skeletal beams sufiiciently attests 

 the presence of the chamber-layer iîiside the beams. Of the rest 

 of the parenchymalia belonging to the outer set, a small portion 

 runs in a thin loose layer on the external choanosomal surface 

 over the blind ends of the chamber-layer evaginations, while 

 a by far larger portion traverses in all directions the interspace 



