124 Prof. W. Thomson on the " Vitreous " Sponges. 



Another set from ApJirocallistes are especially interesting 

 (woodcut, a) : they consist of a lengthened shaft ending in 

 a small expansion, from which spring four equal branches, 

 each terminated by a little knob. No doubt these are the 

 separated branches of a complex hexradiate spicule closely re- 

 sembling those figured by Bowerbank (British Sponges, vol. i. 

 figs. 190-192) from species of the penultimate genus of our 

 series, Dactylocalyx. 



General Structure of the Sponges. 



The netted walls of the two species of Habrodictyon are 

 formed of a loose open network of fascicles of comparatively 

 short fusiform spicules, the fascicles loosely bound together by 

 the soft sarcode. The bundles curve irregularly in all direc- 

 tions, so that the network is quite irregular. No distinct 

 bundles of long fibres pass longitudinally from end to end of 

 the sponge, or transversely roimd it as in Euplectella. In H. 

 corhwula the upper truncated end of the tube is closed by a 

 netted lid, of a denser aggregation of spicules than that which 

 forms the general wall ; while in H. speciosum the general wall 

 'extends uniformly over the enlarged end of the tube without 

 any change of structure. 



The spongy portion at the base of Hyalonema resem- 

 bles closely in minute structure the wall of Hahrodictyon. 

 The spicules are nearly of the same form, and are arranged 

 much in the same way ; but the bundles of needles are meshed 

 into a porous conical mass whose parts tend to radiate towards, 

 to be combined with, and to support a bundle of enormous 

 spicules, whose lower portion is twisted into a close, compact, 

 tapering coil in the centre of the sponge, while the upper part 

 of the coil projects a foot above the centre of the sponge, and 

 is frayed out in the water like a glittering brush of glass*. 



In Euplectella^ the long vertical spicules, instead of forming 

 an isolated wisp as in Hyalonema^ are separated into small 

 fascicles, which are spread out symmetrically and connected 

 into a netted tube by ring-like bundles of transverse fibres. 

 The result is a wonderfully beautiful and symmetrical net 

 with square meshes. Over this framework the general sponge- 



* I have not space at present to discuss the curious diversity of opinion 

 which exists as to the relations of Hyalonema. I will only refer the reader 

 to an admirable memoir by Professor Max Schultze, * Die Hyalonemen,' 

 Bonn, 1860, to a second paper by the same author in the * Annals and 

 Magazine of Natural History,' March 1867, and to a paper of my own in 

 the ' Intellectual Observer ' of the same date. 



