328 NATURAL HISTORY. 



every current, and cannot, therefore, ascend a river where the current is always seaward. This 

 explanation will probably account for the absence of many other marine forms of life which one might 

 expect to find amongst the fauna of our rivers. 



The statoblast is no doubt an adaptation to preserve the Sponge from the extreme climatal 

 changes to which fresh water is exposed; thus in Bombay it develops at the time of summer 

 droughts, in temperate climates on the approach of winter. It is worth noticing that the stato- 

 blast of the Polyzoa also occurs only in the fluviatile forms. 



Suberitidince.* In these Sponges the spicules are characteristically pin-shaped, densely aggre- 

 gated together, either in fibres, or matted felt-like. The surface often bristles with the projecting 

 points of radiating spicules (Plate 71, Figs. K, L), but is never provided with a regular spiculous 

 network like that which usually occurs in the Renierinse. 



The large Sponge appropriately named Neptune's Cup (Poterion neptunirf Fig. 9, c), found 

 growing on the coral reefs of the Indian Ocean, and fossil in the English Chalk, is a Suberite. A near 

 relation, possessing the same pin-like form of spicule, is the little burrowing Sponge Cliona, which eats 

 its way into shells, particularly oyster-shells. Shells so infested may often be seen at the fishmonger's. 

 They may be at once distinguished by the numerous round holes which cover the surface. The holes 

 are of two sizes, the larger for the emission of oscular tubes, the smaller, which are much more 

 numerous, for poriferous tubes. On splitting the shell open both are found to communicate 

 with irregularly swollen canals, which are occupied by the yellowish-coloured body of the Sponge. 

 If the oyster, fresh from the sea, be placed in a vessel of cool clear sea-water, a beautiful sight will 

 soon present itself. From the various apertures delicate mobile tubes protrude. Those from the 

 larger end in a single oscular opening ; those from the smaller expand at the end into a conical form, 

 resembling, with the swollen base perforated by numerous little pores, the " rose " of a watering-pot, 

 with the addition that here the margin of the "rose " is fringed with a corona of delicate diverging 

 spicules. The tubes are very sensitive not only to touch, but to the incidence of light, instantly 

 contracting and withdrawing themselves when exposed to powerful sunlight. Currents of water flow 

 into the poriferous tubes, which swing to and fro, seeking the water most to their taste, and from the 

 equally mobile oscular tubes currents briskly escape. In autumn, this sponge-mass will be found 

 crowded with little oval yellowish bodies, about ^th of an inch long, which are the ciliated embryos 

 or larvse of the Sponge. Spicules are already developed in them. The burrows of Cliona occur in 

 fossil shells of the Silurian strata. 



Desmacidince. This is the culminating group of the Monaxonida?, distinguished by the rich 

 variety of its spicule forms. Besides the chief spicules, which are usually bi-radiate (acerate or 

 acuate), there are always present one or more forms of small spicules, C- and S-shaped hamates ; 

 tri-curvates ; equi- and inequi-anchorates, singly dispersed or clustered into rosettes; and trichite- 

 sheaves. It is from this group that the Tetractinellidse have probably been derived. 



Echinonemata.% The skeleton is characterised by Carter as composed of chief spicules lying 

 parallel to form a fibre, which is spined by other (echinating) spicules projecting from it, Schmidt 

 considers the absence of hamates and anchors essential to the definition of the Chalinopsidinse, a 

 group otherwise equivalent to the Echinonemata. Spongin is usually present, cementing the spicules 

 together ; it may increase in quantity, replacing the spicules, which may diminish to a single row, or 

 disappear from the interior, the echinating forms of course persisting. Should they also vanish, the 

 Sponge would become a Cerospongia. 



CJialinidce. The common Chalina occulata of the British coasts is a good example of this 

 group. The skeleton consists of spongin fibre, cored by silicious spicules, which are usually 

 monaxial ; echinating spicules are absent. The relative development of the spicular axis and the 

 spongin wall is very variable, some, like Pachychcdina, approaching the Renierinje by the pre- 

 ponderance of spicules ; others, like Chalina, approaching the true horny Sponges (Cerospongia) by 

 the excessive development of spongin. This family, indeed, links together the Silicispongise and the 

 Cerospongia3, and since its spicules must apparently be formed before the spongin which envelops 

 them, it would appear rather that the Cerospongige were derived from the Silicispongise by loss of 

 spicules, rather than the latter from the former by their acquisition. 



Latin, suler, cork. f Greek, poter, a drinking cup. J Greek, echinos, a hedgehog ; nema, a thread. 



