28 THE VOYAGE OF H.M.S. CHALLENGER. 



the so-called " synajsticula," wliich stretch so frequently between neighbouring spicules 

 of many Lyssacina, binding them into a solid framework, nor finally in those remark- 

 able lattice networks which occur in many Hexactinellida on regions in contact with 

 foreign bodies, but especially where the Sponge has grown on a solid substratum. 

 In individual cases, as, e.g., in the thickened extremities of many anchor spicules, 

 the central canal exhibits a brush-like division into several diverging, blind, terminal 

 branches (PI. III. fig. 29 ; PL XIV. fig. 5). 



The innermost layer immediately surrounding the central canal is called by Claus 

 the " axis cylinder." It is generally distinguishable from the usually many layered 

 outer cortex by its somewhat feebler refractive power and by the absence of lamination. 

 From the behaviour of the spicules when heated, and when examined in polarized light, 

 Max Schultze determined that the individual lameUse were separated from each other 

 by thin layers of an organic substance. After cautious heating, fine brown carbon 

 streaks were seen between the adjacent hyaline layers. By the use of the polarising 

 apparatus double refraction could be demonstrated in the thin intermediate layers, but 

 not in the substance of the lamellae. 



Professor Maly of Graz was kind enough to analyse a number of spicules 

 from the root tufts of a Poliopogon amadou, and has summed up the result in the 

 following note, with which he has favoured me: — "The spicules after being placed in a 

 desiccator, and dried at 105°, still contained 7 "16 per cent, of water in chemical union. 

 They are, therefore, not silicic acid in the mineralogical sense (quartz substance), but a 

 hydrated silicic acid, and therefore resemble opal, in which the amount of water very 

 frequently varies from 6 to 8 per cent." 



In no other group of Sponges is there so great a variety in the form of the skeletal 

 elements as in the Hexactinellida, yet nowhere are the numerous individual forms of the 

 spicules so readily referable to a common fundamental type. 



As was first recognised by Wyville Thomson, and subsequently abundantly confirmed 

 by Oscar Schmidt, Marshall, and others, a system of three equal axes intersecting at 

 right angles is the fundamental structure of the skeletal parts in all Hexactinellida, and 

 that both in regard to the spicules united into a continuous framework, and those which 

 lie isolated. Though the disposition of the axes is thus in fundamental agreement with 

 that of the regular crystallographic system, the structure of the spicules is by no means 

 the result of crystallisation. The silicic acid occurs here in an absolutely amorphous 

 condition like that of the opals with which they are, chemically, so closely related. 

 Only in a few fossil specimens does the existence of double refraction indicate a secondary 

 transformation into the crystallised state. 



The axes of the individual skeletal elements are usually quite distinctly indicated by 

 the disposition of the principal rays in relation to the point of intersection, but frequently 

 they can only be demonstrated by examination of the central canals. 



