PORIFERA. 



69 



direction of the aqueducts through the sponge, 

 and the position of their orifices or oscula on 

 the surface, is very much a matter of chance, 

 and that their formation is the result of a 

 mechanical cause liable to be diverted from 

 its course by exterior circumstances. If we 

 follow the growth of a sponge, we may feel 

 still more confirmed in this view. The species 

 begins as a spot-like crust of uniform texture, 

 porous throughout, and nearly equally so. In 

 this primitive, homologous condition, there is 

 nevertheless a perfect circulation, a current 

 which seeks the interior, and another which 

 flows from it, to mix with the circumfluent 

 medium. As the sponge grows in extent and 

 depth, the space for imbibition is enlarged, and 

 the centrifugal water, in its efflux, flowing at 

 first into one and then into more currents, 

 these gradually make for themselves channels 

 in the cellular texture, the fibres of which are 

 pushed aside, and prevented, by the continu- 

 ance of the stream, from again encroaching on 

 its course. The channels increase in number 

 with the continued increase of the sponge, and 

 as it cannot but happen that they shall oc- 

 casionally open into and cross each other, we 

 have a wider canal formed by the additional 

 flow of water into it. Such of these canals 

 as reach the surface, soon effect for themselves 

 an opening there; for the current in it pushes 

 against the superficial coat that opposes its 

 efflux, and gradually thins and loosens its 

 texture until this ultimately disappears leaving 

 a fecal orifice or osculum. This is frequently 

 a simple circular hole ; but often, on looking 

 within the outer rim, we notice in the funnel 

 from two to five lesser oscula united together, 

 which are the openings of so many canals that 

 have united there ; and sometimes we find 

 spread within the osculum, or over its mouth, 

 a net work of finer texture than the rest of the 

 sponge, but otherwise of the same nature and 

 composition." 



" Such, we believe, to be the manner in 

 which the canals and oscula are formed, and 

 hence we cannot give our assent to the notion 

 that the net-work spread over or within them 

 is intended as a ' wise provision '* against the 

 intrusion of noxious animals, or other foreign 

 bodies within the sponge, which seems indeed 

 to be sufficiently protected at these orifices by 

 the efflux of the currents passing continually 

 from them. Neither can it be supposed that 

 the position and elevation of the oscula have 



* " When we cut a thin piece of the surface of a 

 living sponge, and look down thi-ough one of its 

 pores with the reflecting microscope, we perceive, 

 immediately beneath the projecting spicula which 

 defend the pore, a very delicate network of gelati- 

 nous threads thrown over the entrance of the tube. 

 This piece of structure is so fine as to be perfectly 

 invisible to the naked eye ; it consists of five or six 

 threads which pass in from the sides of the tube, to 

 be connected with a central mesh ; so that there 

 are five or six meshes thus formed ; and while this 

 soft apparatus is beautifully defended by the pro- 

 tecting spicula of the pore, it serves still further to 

 guard the interior of the animal from the smallest 

 particles of sand or the minutest visible animal- 

 cules." Grant, Edinb. Phil Journ. 



any foreseen relation with the situation of 

 the sponge in the water. When, according 

 to Dr. Grant, this production spreads level on 

 a rock with an upright aspect, the oscula are 

 raised into crater-like cones, to enable the 

 sponge to clear itself of the excrementitul 

 matters carried out by the centrifugal streams ; 

 but when it hangs pendent from the rock the 

 oscula do not rise bevond the surface, because 

 the necessity of ejecting excrementitial matters 

 to a distance does not exist. This is to be- 

 stow a foresight and instinct on the sponge 

 which even the followers of Lamarck would 

 hesitate to give it, and which we may safelv 

 deny it to be possessed of. The form of the 

 oscula depends entirely on the texture of the 

 species, and on the force of the effluent cur- 

 rents. If the texture be loose and fibrous it 

 yields easily, and the oscula are level, or nearly 

 so : if more compact the skin is pushed be- 

 yond the surface into a papillary eminence ; 

 and if too firm and dense to yield to the pres- 

 sure behind, they fall into a level condirion. 

 They are also liable to be modified in some 

 degree by external forces, for the littoral 

 sponge, which, in a sheltered hollow, or fringed 

 pool, will throw up craters and cones from its 

 surface, may be only perforated with level os- 

 cula, when it is swept over, and rubbed down 

 by the waves at every tide." 



Reproduction. The following are Pro- 

 fessor Grant's recorded views upon this sub- 

 ject. " Every part of the gelatinous matter 

 (which invests the skeleton of the sponge) is 

 covered with minute granular bodies, which are 

 distinctly seen in every species of sponge by the 

 weakest magnifier of the microscope. These 

 granular bodies are represented in the plates 

 of Donati of a spherical form, adhering to the 

 quadriradial fibres of what he has named the 

 Alci/oniinn primum Dioscoridis, They are 

 quite invisible to the naked eye ; they escape 

 along with the gelatinous matter, and coin- 

 pose the greater part of it ; they are connected 

 with each other by the gelatinous matter, 

 and probably by the same medium, have some 

 connection with the spicula, along which they 

 are placed. No part in the organization of a 

 sponge is more constant and obvious than 

 these granular transparent bodies, lining the 

 interior of every canal from the p>res to the 

 fecal orifices. Their form is not quite spherical, 

 but somewhat lengthened and ovoidal, and 

 they are always attached by one extremity to 

 the gelatinous matter, while their opposite end 

 is seen to project free into the cavity of the 

 canals. Through the greatest magnifier of the 

 microscope no difference can be detected in 

 their forms in different species of sponge ; they 

 all appear to be enlarged, and round at theirfree 

 projecting extremity, and, when watched with 

 attention, we distinctly see that they possess 

 some power of spontaneous motion both when 

 in connection with the sides of the canals and 

 when lying isolated at the bottom of the 

 water. The ova of the sponge are quite 

 visible to the naked eye, and are seen dissemi- 

 nated through the whole texture of the sponge 

 in the winter season. They are bodies of a 



F 3 



