120 Prof. W. Thomson on the " Vitreous''' Sponges. 



— an assemblage of the most beautiful, the most singular, and 

 the rarest of marine productions. 



Geneeal Chaeactees of the Geoup. 



Condition of the Barcode. 



From its essential simplicity and the want of any true struc- 

 ture, the sarcode of the glassy Sponges cannot be expected to 

 afford any very marked distinctions ; still even this element 

 seems to differ in certain characters from the condition in 

 which we find it in the other orders of Sponges. It is small 

 in quantity, very soft, probably semifluid, extending in a thin 

 layer over the fascicles of siliceous needles and over the sili- 

 ceous framework. It appears to contain no trace of the dif- 

 fused granular horny matter with which the more consistent 

 sarcode of the Halichondrida is so often loaded. When a 

 vitreous Sponge is dried (and all the specimens which have 

 yet reached Europe are in a dry state), the whiteness of the 

 skeleton is barely masked by the pale yellow film which re- 

 presents the contracted animal matter. Most of the specimens 

 of EuplecteUa in the market have been bleached ; but some of 

 them, which may be recognized by their pale fawn-colom*, are 

 merely dried ; and if a portion of one of these be steeped for a 

 short time in a warm weak solution of caustic soda, the sarcode 

 softens and expands, and may be examined under the micro- 

 scope with tolerable success. It is generally almost transpa- 

 rent, with here and there scattered endoplasts and minute 

 compound granular masses. Among the meshes of the sponge- 

 network, and everywhere except where it is extended (as in 

 Hyalonema and EuplecteUa) over the sm*face of enormously 

 long separate needles, the sarcode contains abundance of ex- 

 tremely minute spicules, scattered through it singly or aggre- 

 gated in groups. These spicules, as we shall see hereafter, 

 are often complicated in form and ornament, and are highly 

 characteristic of the order and of the several genera. 



The Siliceous Skeleton. 



In Hahrodictyon and Hyalonema the skeleton is composed 

 entirely of separate siliceous spicules of various forms, inter- 

 woven in fascicles and connected by the thin sarcode layer, or 

 scattered irregularly among the fascicles of spicules. In Eu- 

 plecteUa, Aphrocallistes, Dactyhcalyx, and Farrea, certain 



ing his lovely specimen, upon which Professor Owen founded the species 

 EuplecteUa cucumer. I can have no doubt that this is merely an example 

 of E. aspergillum of a rather unusual form, which has attained its full 

 size, hut in which the raised spiral crests are as yet imperfectly developed. 



