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Observations on the Structure and Functions of the Sponge. By R. E. GRANT, M. D., F. R. S. E., F. L. S., M. W. S., ·Honorary Member of the Northern Institution, &c. Communicated by the Author. Concluded from the preceding Volume, p. 351. (With a Plate.)
THE silicious and calcareous spicula above described are grouped into strong fasciculi, which are disposed around the internal canals of the sponge, in the order best calculated to defend these passages from compression, and from the entrance of extraneous bodies, and likewise to form between the canals certain interstitial spaces for the development and exit of the ova. Like the hard parts composing the skeleton in other animals, these earthy spicula are maintained in their relative situations by a tough ligamentous matter, distinct from the other soft parts of the sponge. In the horny species, however, where the axis is composed of cylindrical tubular horny fibres, ramified and continuous throughout the whole body, this connecting cartilaginous matter appears to be unnecessary, and, from the examination of dried specimens, it appears to be altogether wanting. The examination of the living properties of the axis in the horny species forms a subject of curious and interesting inquiry, which must be left to those who have opportunities of observing them alive in warmer latitudes, as they do not seem to inhabit the British shores. The dried filaments of the S. fistularis, Lam. when viewed through a powerful microscope, appear to consist of one continuous ramified tube, whose central cavity (Pl. II. Fig. 19. b) is entirely filled with a dark opaque granular matter, which does not consist of spicula, while the sides of the tube (a) are transparent and amber coloured like common cat gut. In the S. officinalis, where the filaments are much finer, the sides of the tube (Fig. 20. a) have the same colour and homogeneous appearance, but the central cavity (b) appears empty. Mr Ellis states,
that, in the branched species, the central cavities of the horny filaments are filled with a soft white matter, and that they terminate by distinct apertures on the surface of the body; and he considered these cavities as undoubtedly the habitations of animals of a particular kind, (Hist. des Cor. p. 94). The confirmation of this opinion, by accurate experiments, would establish a very striking distinction between these elastic species and the more friable earthy sponges of our own shores, and would point out a remarkable approximation in these highly organised species to the polypiferous axis of tubulariæ, sertulariæ, and other keratophytes. In all the calcareous sponges which I have hitherto examined, we invariably find triradiate spicula, which are completely enveloped in the connecting matter, and are employed in forming the bounding fasciculi of the pores. Besides these complicated spicula, we frequently find a second and simpler form of spiculum, one extremity only of which is immersed in the connecting matter, while the other end, projecting free from the surface, defends the entrance of the pores and orifices. Thus, in the S. compressa (Fig. 23.), the bounding triradiate spicula (Fig. 11.), of various sizes, are found enveloped in the tough connecting matter around the pores, the defending clavate spicula (Fig. 12.) have their straight tapering portion immersed in the connecting matter, while their curved extremity hangs free over the entrance of the pores. In the S. coronata the connecting matter seems to cover entirely the bounding triradiate spicula (Fig. 17.); and only the thick obtuse extremity of the needle-shaped defending spiculum (Fig. 18.) is immersed in it, while the tapering pointed end hangs free over the pores and fecal orifice. I have never observed a combination of calcareous and silicious spicula in the same sponge, nor any kind of spiculum in the horny species. Two distinct forms of spicula are very seldom observed in silicious sponges, though they are frequent in the calcareous species. In the Spongia ventilabrum, Lin., besides the long waved silicious filament (Fig. 5.), we observe a distinct needle-shaped spiculum obtuse at one end, and tapered to a point at the other, (similar to Fig. 18). In the S. pilosa, Mont., besides the long straight fusiform spiculum, we observe a shorter curved spiculum, of equal thickness throughout, and rather obtusely pointed at both ends, like that of the
Spongilla friabilis (Fig. 1.), but larger. In general, however, the only difference observed among the silicious spicula of the same individual is a great variety in their size. Donati not only observed that the hard spicula of the Tethya sphærica differed remarkably in size, but likewise, that they were bound together by a peculiar fleshy or tendinous matter, (Mar. Adr., p. 62). In the S. coalita, besides the slender curved fusiform spiculum (Fig. 2.), we observe a long thick spiculum of the same form, which extends along the sides of two or three successive pores, and contributes much to their strength in a species peculiarly liable to have the diameter of these passages disturbed from the flexibility of its branches, and their erect position at the bottom of the sea.
At the approach of death, and during putrefaction, the soft gelatinous or cellular matter of the S. panicea escapes plentifully from every opening of the body, and drops down like the ropy transparent colourless matter of an egg, without loosening, in the slightest degree, the connecting matter of the spicula, or altering perceptibly the form of the skeleton. When we extract, by strong pressure, the cellular matter from the S. coalita, S. tomentosa, &c. we obtain a very tough leathery substance, composed of spicula firmly bound together by the cartilaginous matter, and retaining the original colour and form of the sponge. By repeatedly and strongly agitating a thin portion of the recent S. papillaris in fresh water, and then examining it under a powerful microscope, we find that the cellular matter has been entirely washed away, and the spicula are left imbedded in a transparent homogeneous tough matter, which retains its original colour and form unaltered. This connecting matter tears like a piece of cartilage, emits a fishy odour when burnt, dissolves without effervescence in nitric acid, contracts much, and acquires an amber colour by drying, and becomes very brittle in the dried state, probably alone from the earthy spicula it contains. There seems, therefore, to be a distinct matter in the earthy sponges for connecting, and probably secreting, the spicula of their skeleton. The dried preparations of this animal, preserved in museums, owe their form and stability to this tendinous connecting substance, and, from its close resemblance in the dried state to the amber coloured filaments of horny species, it is pro