SPONGES 105 



conditions that prevail are the exact opposite of those with which the 

 arrangement of the spicules is usually correlated. 



The arrangement of the spicules in the diverticula and oscular 

 tubes of Leucosolenia (Fig. 73) foreshadows, and gives a clue to, 

 the plan of the skeleton in the Heterocoela. Taking the simpler 

 syconoid type as the starting-point for this group, we find that at 

 their first origin the ciliated chambers or radial tubes arise as 

 simple diverticula of the gastral cavity, differing only from those 

 of Leucosolenia in that they are more numerous and retain a more 

 simple unbranched condition, not giving rise to new oscula. Each 

 radial tube has its wall supported by spicules forming a special 

 tubar skeleton, distinct as a rule from the more internal gastral 

 skeleton both in arrangement and composition, and representing, there- 

 fore, in the latter respect a slight advance in specialisation upon the 

 state of things seen in Leucosolenia. In the more primitive types the 

 organisation scarcely advances beyond this point, except for the 

 formation round the osculum of a special peristomial skeleton, con- 

 sisting for the most part of elongated monaxons, and of a peduncular 

 skeleton in the stalk. But with fusion between the distal ends 

 of the radial tubes, to form a cortex, a special skeleton becomes 

 differentiated in this region also, so that the skeleton of the body 

 wall in a typical Sycon consists of three layers : (1) most externally 

 a cortical skeleton, which is said to be " smooth," when it consists 

 of triradiates only, and " hispid," when it contains monaxons, with 

 or without triradiates ; (2) a tubar skeleton composed of triradiate 

 systems, some of which may develop a gastral ray; (3) most 

 internally a gastral skeleton, composed mainly of quadriradiates 

 (Figs. 68, 69). 



The tubar skeleton shows two distinct types of organisation 

 known respectively as the articulated and the non-articulated. In the 

 former, which is the more primitive, and directly comparable to the 

 state of things in Leucosolenia, each radial tube has its wall supported 

 by sagittal triradiate systems arranged in several series, each with 

 the unpaired posterior rays pointing towards the distal extremity of 

 the chamber (cf. Figs. 74, a, and 73). In the non-articulated type 

 of tubar skeleton there is but a single series of these triradiates, 

 each one situated near the base of the radial tube and sending a 

 greatly elongated posterior ray towards the apex, which meets, and 

 runs parallel to, a similarly hypertrophied lateral ray (Polejaeff) of 

 a triradiate of the cortical skeleton (Fig. 74, b}. By interlocking of 

 these two systems of modified spicule rays the chamber acquires a 

 firm and rigid skeleton. 



With the evolution of a leuconoid type of canal system the 

 pronounced radial structure seen in the Sycons becomes lost, and 

 the elongated radial tubes become very much shortened and con- 



