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GROWTH 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 re- 

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

 forming the mass itself, and not, as in other instances, arranged as spiculae 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. 



BEFORE taking leave of these interesting beings, we must glance rapidly at tne 

 method by which they distribute themselves so widely and increase with such marvel- 

 lous 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 of 

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

 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 often 

 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. 

 Their shape is very like that of a pear ; and as they are wholly covered with cilia, ex- 

 cept 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 tITemselves to the spot, 

 from which they never afterwards can move. The rounJed 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 ac- 

 tion 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 spiculae become visible, and, after three weeks or 

 a month have passed away, the spiculas 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 odor of their decay. 



