772 GEOWTH OF SPONGES. 



but exceedingly shallow, and on the inside it is tolerably smooth, becoming rougher and 

 deeply grooved on the outer surface. It has a peculiarly rough feel to the touch, almost 

 exactly resembling the well-known rasping effect produced by rubbing pumice-stone 

 upon the skin ; and it is in consequence of this resemblance that it has gained its 

 popular title. 



The peculiarities of this very beautiful Sponge consist in the following distinctive 

 characters, the most remarkable of which is its being formed entirely of silex, the 

 reticulate structure of the mass being composed of transparent glassy tubes, the silex 

 forming the mass itself, and not, as in other instances, arranged as spiculse in the horny 

 membranes ; consequently, it is perfectly rigid and sonorous when struck. 



When viewed under a microscope of about seventy-five diameters, the net-like meshes 

 are seen to be composed of beautiful glassy tubes, uniting one with the other in every 

 direction, the external surface of the cylinders having a rugged aspect. The newest or last- 

 formed portions appear to emanate from centres, and at certain distances from spherical 

 knobs, from which straight tubes again arise, thus forming the net-like mass. 



BEFOEE taking leave of these interesting beings, we must glance rapidly at the method 

 by which they distribute themselves so widely and increase with such marvellous 

 rapidity. 



It will be remembered that the soft animal matter of which the true Sponge is formed 

 is composed of multitudinous bodies which closely resemble the Amoebae, and many o' 

 which are furnished with thread-like cilia. In certain months of the year, which in ou 

 country are generally found to be October and November, a vast number of very minute 

 yellowish particles are to be seen studding the body of the Sponge. They are not ofter. 

 seen near the surface, but are gathered plentifully within its multitudinous cells. Small 

 as are these yellow particles, they are formed of many eggs, or " gemmules," as they are 

 called, of the Sponge, which gradually increase in size, and at last are expelled from the 

 larger orifices, and thrown at random into the wide sea. 



There they are, flimsy, minute, shelterless, feeble, and apparently helpless. Small, 

 however, as they may be, they still possess the power of transporting themselves through 

 the water by means of the cilia with which their bodies are abundantly studded. Theii 

 shape is very like that of a pear ; and as they are wholly covered with cilia, except the 

 narrow end, it is evident that their larger end must always be in front. They lead a free 

 life for several days after their expulsion from the parental home ; and even in this early 

 stage some indications of the future framework are to be seen. 



After the lapse of some little time, these gemmules meet with some object which 

 affords them a suitable resting-place, and accordingly affix themselves to the spot, from 

 which they never afterwards can move. The rounded body soon becomes flattened, as it 

 adheres with a close grasp, and spreads itself into a nearly circular film. The cilia still 

 exist on the upper surface of this film, but the effect of their action is then not to propel 

 the Sponge, but to create a current of water which can pass over it. 



As time passes on, the distinctive spiculse becoms visible, and, after three weeks or a 

 month have passed away, the spiculse have been gathered into little bundles, which by 

 their arrangement tend to preserve the shape of the Sponge and to keep the orifices open 

 The little being now spreads rapidly, by a process which much resembles the subdivision 

 of the Infusoria, and the whole mass of the Sponge is evidently composed of a vast 

 number of the Amoeba-like bodies which have already been described. Thousands upon 

 thousands of these gemmules are passed out into the sea from every Sponge that inhabits 

 its waters ; and the only wonder is, that, in consequence of such marvellously prolific 

 properties, the Sponges do not swarm to such an extent as to fill the whole seas, and 

 poison the entire earth with the odoui of their decay. 





