70 



PORIFERA. 



yellow colour, somewhat translucent, pear- 

 shaped, tapering more or less at their narrow 

 end in different species ; their whole outer 

 surface is covered with delicate projecting 

 cilia, and when viewed through the micro- 

 scope, in connection with the parent, we see 

 that the rapid vibration of these cilia produces 

 a distinct current in the water immediately 

 around them, flowing always from their 

 rounded free end towards their tapering fixed 

 extremity, thus assisting the small granular 

 bodies in producing the currents of the sponge 

 during the period of their attachment to the 

 body. They separate from the canals, and are 

 propelled through the fecal orifices early in 

 spring. None of these ova are seen in the 

 sponge in summer, though we can detect no 

 difference in the velocity of the currents at 

 that period. For some time after they are 

 propelled from the interior of the sponge, they 

 swim about by means of the cilia on their 

 surface, and exhibit all those extraordinary 

 phenomena of spontaneous motion which 

 Cavolini, nearly half a century ago, discovered 

 in the ova of the Gorgonia and Madrepore. 

 They at length fix themselves, like the ova 

 alluded to, on a spot favourable to their 

 growth ; they lose entirely their original form, 

 and become a flat transparent circular film 

 through which horny fibres shoot ; they soon 

 spread, and assume a form similar to that of 

 the parent." * 



Gemmules. Mr, Bowerbank has given the 

 following description of the gemmules of 

 Halichondria Johnstonia. " The gemmules of 

 this sponge are dispersed in great abundance 

 throughout every part of its substance ; they 

 are of an oval form, the longest diameter being 

 2T7 tn and the shortest, the ^^th of an inch. 

 They vary considerably in size, but the above 

 are their average dimensions. When seen by 

 direct light, with a power of 100 linear, they 

 appear of the same colour as the surrounding 

 fleshy matter ; but when viewed as transparent 

 objects they assume an iron or slate-gray 

 colour, having their surfaces closely studded 

 with minute papillae, which are produced by 

 the projection of the points of numerous 

 very small spicula, which are imbedded in the 

 crust or shell of the gemmule, and are dis- 

 posed in lines radiating from the centre to the 

 circumference of the body." 



The form of these minute spicula is ex- 

 ceedingly various ; but the best developed ones 

 appear conical, having their bases towards 

 the centre of the gemmule, and their apices 

 slightly elevating the parts of the outer in- 

 tegument immediately above them. The mode 

 of disposition of these spicula is best observed, 

 when a small portion of the sponge has either 

 been treated with boiling nitric acid, or by 

 incineration in the flame of a lamp. The 

 dissolution of the gemmules is not effected by 

 either of these agents, and, to view them with 

 the greatest effect, they should be gently 



* Edin. Phil. Journ. ; and Edin. New Phil. Journ. 

 vol. ii. p. 128, &c. 



triturated with a little water between two 

 pieces of glass, until some of them be broken 

 into small pieces. In these fragments, the 

 spicula may be seen in situ, cemented together 

 apparently by siliceous matter, which appears 

 to abound in the outer integument of the 

 gemmule. Upon measuring some of these 

 minute spicula in situ, Mr. Bowerbank found 

 the average length to be ^Vo of an inch, or 

 about equivalent to the diameter of a disc of 

 human blood, and their average thickness the 

 jtfcrs of an inch, so that they are of exceed- 

 ing minuteness as compared with those found 

 in other parts of the same sponge. 



The propagation of Tethea is by means 

 of sporules or gemmules generated within 

 the fleshy substance. The sporules, ac- 

 cording to Dr. Johnston, resemble the parent 



Fig. 73. 



, Oviform bodies found immersed in the paren- 

 chyma of Tethea Cranium magnified; b, one of 

 these bodies viewed through the microscope after 

 compression between two plates of glass. (After 

 Johnston.) 



sponge in miniature ; but they have no dis- 

 tinct rind or nucleus, being composed of 

 simple spicula woven together by the albu- 

 minous matter ; and there seems no way of 

 escape for them, except by the dissolution of 

 the body of the parent sponge, which most 

 probably is an annual production. " The natu- 

 ralist*, who believes that sponges have an 

 affinity with the fungi, will see, in these par- 

 ticulars, a correspondency which may strength- 

 en his belief. The Tethea, he may say, is the 

 sea's copy of the earth-born Scleroderma, and 

 he may remind us that, like the sporules of 

 sponges, the sporules of fungi are equally 

 locomotive. The Chaos fungor urn of ^Linnaeus 

 is thus described : " Habitat uti semen Ly- 

 coperdiy Agarici, Boleti, Mucores, reliquorum- 

 que fungorum, in sua matre usque dum disper- 

 gatur et in aqua exclusum vivit et moritur, 

 demutn figitur, et in fungos excrescit. Zoophy- 

 torum metaphorphosis e Vegetabiliin Animale 

 fungorum, itaque contrario ex Animali in Vege- 

 tabile." Syst. p. 1326. 



The admissibility of sponges into the animal 

 series is, indeed, extremely problematical, and 

 we doubt not that among naturalists of the 

 present day the balance of opinion would be 

 unfavourable towards retaining them in the 

 rank which they at present occupy in zoolo- 

 gical classification. 



(T. Ri/mer Jones.) 



* Johnston, p. 82. 



