48 



INVERTEBRATE AXIMALS. 



Such, then, lire the general phenomena exhibited by any sponge, 

 and the principal point "which still remains for consideration is the 

 nature of the hard structures which form the " skeleton " of the 

 Sponges. Some sponges (such as Halisarca) have no skeleton ; and 

 when a skeleton is present, it varies much in its constitution. In 

 the sponges of commerce {Euspongia) the skeleton consists solely of 

 hollow horny fibres, which are interlaced with one another so as to 

 form a close reticulation. In many other common sponges, a horny 

 skeleton is also present, but it is associated with, and sometimes 

 largely rejilaced by, microscopic needles or "spicules" of flint. 

 These spicules (fig. 24, B) are of very various shapes in different 

 sponges, and they may be either scattered through the sponge-flesh, 

 or contained in the interior of the horny skeleton-fibre, which they 

 often extensively replace. In the so-called " Siliceous " Sponges, or 

 " Glass Sponges," no horny fibre is developed, but the whole skele- 

 ton is composed of spicules of flint, which are often soklered to- 

 gether so as to form a continuous framewoi'k, which looks like 

 a lattice of spun glass. The beautiful Venus's Flower - basket 

 {Euplectdhi) and the Bird's-nest Sponges {Holtenia) are examples 

 of such sponges. Lastly, in the so-called 

 "Calcareous Sponges," such as O'raiitia (fig. 

 22, A), the skeleton is composed of aggre- 

 gated, b"ut disunited, spicules of lime, in- 

 stead of flint. These spicules mostly have 

 a characteristic three-rayed form. 



As regards the reproductive process in tlie 

 Sponges, it will be sufficient to state very briefly 

 the leading phenomena which have been observed 

 in the Fresh-water Sponge [Spongilla fiuviiitilis). 

 If a specimen of Sj}onriilla be observed towards 

 the approach of winter, its deeper portions will be 

 found to be filled with numerous small rounded 

 bodies like seeds, which have been called " gem- 

 mules." Each gemmule (tig. 25, a h) exhibits at 

 one point a small aperture or "hilum" ; and is 

 found to be composed of a leathery membrane, 

 surroujide^l by a layer of sarcode, in which are 

 embedded a number of spicula. These spicula 

 are called ''amphidiscs," and consist each of a 

 central rod or axle cairying a toothed wheel or 

 disc at each end {fig. 25, c). In the interior of 

 the capsule thus formed is a mass of cells, of which the central ones contain 

 numerous reproductive germs. When the spring comes, these masses are dis- 

 charged into the water through the aperture or hilum of the gemmule, and 

 become developed into fresh Hj)onrfill<B. The above is a non-sexual method of 

 reproduction ; but it is believed that most of the marine Sponges exhibit sexual 



Fig. 25. — a Gemmule of 5po?i- 

 (jilla ; li Hilum ; h Bia- 

 gi-anuiiatic section of tlie 

 geiniiuile, showing the out- 

 er layer nf aniphldiscs and 

 the inner mass of cells ; c 

 One of tlie amphidiscs seen 

 in profile. 



