48 



I.WEUTEDKATE AxXLMALS. 



Such, then, are the geneial jthenoniena exhibited by any sponge, 

 and the principal point whicli still remains for consideration is the 

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

 Sponges. Some sponges (such as I/a/isarcn) have no skeleton ; and 

 when a skeleton is present, it vaiies much iu its constitution. In 

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

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

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

 skeleton is .'dso present, Imt it i.s associated with, and sometimes 

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

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

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

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

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

 "Glass S|)onges," no h<irny llbre is develoijed, but the whole skele- 

 ton is comjiii.sed of s[iicules of flint, which are often soldered to- 

 gether So as tu form a tontinuons framework, which looks like 

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

 {Euphctt'liii) and the Binl's-nest Sponges (/loltenia) are examples 

 of such sponges. Lastly, in the so-called 

 "(.'aleareous Sponges," such as Grantia (&g. 

 2-2, A), the skeleton is composed of aggre- 

 gated, but disunited, sj>icules of h'me, in- 

 stead of flint. These sp'cules mostly have 

 a charactei'istic three-i-ayed form. 



As rL'garils tlie rei)roductive proce.ss in the 

 Simiiges, it will be suHicieut to state very briefly 

 the leading plieuomena whicli have been observed 

 in the Fre.sli-water Sjionge (Simii'jiUa fluriatUis). 

 If a speriinen of Sjjfm;jllla be observed towards 

 tile approach of winter, its deejier portions will be 

 Ibiiiid to be Hlh'il with nnnierons small rounded 

 bodies like scetis, whiidi have been called " geni- 

 liiuh-s. " Each irelliluule (tig. 2.'i, a h) exhibits ;it 

 one point a slhall ai)i'i'tu]-e or "hilinn"; and is 

 found til be coMiiiosed of a leathery ]nenibr.ine, 

 surnimnled by a layiT of sarcode, in which are 

 cnibcdilcd a number of spirnla. These spicula 

 arc cilled " anipliidiscs," and consist each of a 



/; 



*# -V ** *i-t' 



di 



ral 

 at 



a]isnlc 1lins Ion 

 ■runs rcpro.inci i 



cl'ins When 

 ehar:jed iiil,, (lie w.dcr throii-h the 



I'cer I,'\e|,,|ied ill!,, fresli ,S/,f. »,////,( 



reliroduelion ; but it i, liclievcd that n 



id or ax'e carrying a toothed wheel or 

 ncli end [\\[S.. 2'k c). In the interior of 

 cells, nf wliich the central ones contain 

 ilic spring I'onics, these masses are dis- 

 ■■q. crime or liiluin of tlie gemnmle, and 

 . The nbiivc is a non-sexual method of 

 lostof Ihc iiniiine Siionges exhibit sexual 



