OCR Text |
Show 12 ON CLASSIFICA'l'ION. is a most ronutrkab1e circumstance that though those animals arc abundant, and are constantly nndor observation, we arc still in doubt upon that essential point,-still uncertain whether thoro nuty not Lo so1uo phase in tho cycle of vital phenomena of tho Rhizopoda with which we are uHacquainted; and, under those circumstances, a perfect definition of tho class cannot oven Lo atten1pted. The next division is the group of tho SroNGIDA, which exist under such multitudinous fonns in both salt and fresh waters. Up to the last few years ·we were in tho smno cuso, with respect to this class, as with tho Gregar'inicla and tho Rhizopoclct. Somo zoologists even have been anxious to relegate the sponges to the vegetable kingclon1 ; but the botanists, who underf4tood their business, refused to haYe a11ything to do with the intruders. And the botanists were qnite right; for tho discoveries of late years have not loft the slightest doubt that the sponges aro animal organisms, and animal orgnni~1ns, too, of a very considerable amount of cmnplexity, if wo n1ay regard as cmnplex a structure which results fr01n the building np and nwssing together of a nu1nber of similar parts. 'rhe great 1najority of the spongeR form a skeleton, which is composed of fibres of a horny texture, strength noel by needles) or spicula, of silicious, or of calcareous, n1attor ; and this framework is so cormceted together as to fonn a kind of fibrous skeleton. This, however, is not the essential part of tho animal, which is to Lo sought in that gelatinous substance, which invests the :fibres of the skeleton during life, and is traversed by canals which open upon the surface of the sponge, directly or indirectly, by many minute, and fewer large, apertures. If I may roduco a sponge to its si1nplost expression-taking the cmnrnon Spongilla, for exmnplo, of our fresh wators,-tho structure-removing all cmnplexitics, and not tron1ling ourselves with the skeleton, Lccause that has nothing to do with ·what we are now considering-1nay be represented Ly tho diagram (A, Fig. 3). There is a thin snporficial lavnr (a) formed entirely of a number of the so-called 8J:>Ollgo particles, or nltilnate con1ponents of tho living suLstanco of tho :::;pongc, each of which is similar to an AmCEba, and contains a 1nwleus. These OHICGi\HlNJD.A, HHIZOPOT>.A RPON(HJ).,\, .\ Tl) lNFV~OHJA. JB ([ {/ c Fig. 3.-A, IIypothC'tic·al ~ection ot n 8JIOtl!]i!lrt; a, supc1 licial layc>r; b, inh:tlPnt npHtures; c, ciliated chambcri>; d, an exhalrnt aprrturC'; e, dc•(•pcr suhsta ncc of the RpongC'. The nJTO\\'S inclicate thr clir diem of the rttiTC'nt". B, A small spong-r with a single exhalent apcrtnrc, sN•n from ahon ( artcr LicLerJ,iillll); a, inlnl Pnt :l]•l'rt nn•s · c, ciliated chamhC'rs; d, rxhalC'nt apc1 I nrc. C', a ciliatccl chamber. · D, a ii·pp~ swimming ciliated <'mhryo. · |