In speaking of this propulsion of the sea water through the IIalichondria, in which genus it has been principally observed, the crustaceous species being best adapted for the study of its phenomena, Dr. Johnston re marks+, " A single observation is sufficient to convince us that this circulation has no thin', in common with that of higher animals, but it has some analogy surely with that imbibition and influx of water into the body of most radiated and mollitscous animals which takes place through the skin and through certain canals, which Della Chiage has de scribed and figured as their aquiferoua system. The canals in both cases are not vascular tubes with membranous parietes, but rather fur rows, excavated in the flesh or substance of the body, and leading into wider channels equally unlined. They have in common a direct communication with the circumfluent water, which alone ever flows in them, and the en trance of this water seems to be, in a great measure, or entirely, independent of the will of the animals ; but the polypes and mollusca only have the power of expelling it when they choose by the contraction and compression of the parts which the canals traverse. There is, however, a wider difference in the arrangement of the aqueducts, — in the Itadiata and Mol lusca, the pattern is the same in every in dividual of each species, but in the Sponges it has no constancy,— so that in no two specimens of the same kind do we ever find the arrangement to be exactly alike.
This inconstancy seems to prove that the 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 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 excrementitial matters carried out by the centrifugal streams ; but when it hangs pendent from the rock the oscula do not rise beyond the surface, because the necessity of ejecting excrernentitial 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 safely 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 efliuent 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 condition. 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 Alcyonium minium Diosroridis. They are quite invisible to the naked eye ; they escape along with the gelatinous matter, and com pose the greater part of it ; they are connected with each other by the gelatinous matter, and probably by the same medium, have sonic 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 pores 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 their free 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 yellow colour, somewhat translucent, pear shaped, tapering more or less at their narrow end in different species ; thelr 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 alvvays from their rounded free end towards their tapering fixed extretnity, 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 snread, and assume a form similar to that of the parent."*.