330 



NATURAL HISTORY. 



central oscular tube fringed by whisker-like spicules, and a thick beard-like growth of anchoring 

 spicules, which serve to support it in the soft slimy ooze of the deep sea in which it lives; Hyalonema* 

 (Fig. 9, B), the Japanese glass-rope Sponge, a close ally offfoltenia, but at once distinguishable from it 

 by the spiral wisp or rope into which its anchoring spicules are twisted together ; the upper end of 

 the rope is overgrown with encrusting Palythoa, the lower end frays out by the divergence of the 

 spicules ; deprived of the Sponge the rope may often be seen in private houses stuck topsy-turvey 

 under a glass-shade ; an instance of the perverted ingenuity of the Japanese divers by whom it is 

 obtained and " prepared." 



E^iplecteUa\ (Fig. 9, A), with a framework so exquisitely beautiful in its fairy-like tracery as to have 

 called forth the remark from a distinguished naturalist " this passes the love of woman," has now also 

 become an ornament to glass-shades ; it is a Lyssakine, with spicules so arranged crossing one 

 another as to weave together a thin- walled vase of delicate lattice- work with square meshes. In the 



fresh state, when the skeleton is embedded in the meso- 

 derm, over every alternate mesh, a conical process of the 

 Sponge-wall projects, the other meshes open by a round hole 

 into the interior of the vase. Beneath the poriferous skin 

 (Fig. 16), which is adorned with flesh-spicules, and sup- 

 ported on the distal rays of sex-radiates, thin, threads of 

 mesoderm form an irregular network, in which the flagellated 

 chambers are immersed. These are cylindrical sacs, open at 

 one end, closed and hemispherical at the other ; each is 

 perforated by several pores, through which water can enter 

 from the surrounding lacunar spaces of the mesoderm ; by 

 their open ends they communicate with the digitately 

 branched excurrent canals, which freely open into the 

 central cavity of the vase. The water, which streams suc- 

 cessively through the skin, the flagellated chambers, and 

 excurrent canals into this cavity, escapes partly by the 

 open meshes in the side of the vase, and partly through a 

 netted lid which closes its end. Like so many of the 

 Hexactinellidae which live in the mud of the deep sea, 

 Euplectella terminates below in a tuft of anchoring spicules; but when it is found in shallower 

 water on a hard bottom it becomes attached, and its anchoring spicules abort. 



In the Dictyonina the chief spicules are so disposed that by the overlapping of their rays 

 they form a framework, which afterwards being overrun by silica becomes a continuous net ; the 

 knots or nodes of this net correspond generally with the centre of the spicules, its connecting 

 fibres with two overlapping rays (Fig. 17, B). The spicules are not always so regularly arranged 

 as in the figure ; and in many genera they depart widely from a three-axed form, the rays diverging 

 at all angles, so that one fibre may contain more than two and as many as all six rays. 

 Loose sex-radiate spicules are always associated with the network, and delicate minute or flesh 

 spicules are general throughout the Sponge. The " flower-basket " sponge Dactylocalyx (Fig. 9, E), 

 the earliest discovered instance of a Hexactinellid, is a good example of this group, and Farrea 

 is another, distinguished by the regularity of its square meshes. In some Dictyonina the investing 

 silica fails to completely fill the angles at the centre of the spicules, but stretches across in fine threads 

 from, one ray to another, sketching out the edges of a regular octahedron, with the spicular rays for 

 its axes. This structure was accurately described long before it was understood, by Toulmin Smith, 

 who showed that it characterised the network of the Ventriculites. It is only quite recently that an 

 existing Sponge has been described (Myliusia) in which the same structure prevails (Fig. 17, c). 



The Hexactinellidte inhabit all seas, and are found in deep water, ranging from 98 to 1,591 



fathoms, and probably more. They make their appearance in time very early, remains of a 



Lyssakina (Protospongia) being found in the Lower Cambrian rocks at St. David's, South Wales ; 



both Dictyonina and Lyssakina occur in the Silurian of North America ; in the Carboniferous 



* Greek, hyalos, glass. t Greek, eu, well ; plectos, woven. j Greek, dictyon , a net. 



Fig. 17. A, SEPARATE SPICULES OF A LYSSAKINE 

 SPONGE ; B, SPICULAK NETWORK OF A DIC- 

 TYONINA ; C, DICTYONINE NETWORK WITH 



OCTAHEDRAL KNOTS. (After Marshall and Zittel.) 



