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Show 1873.] DR. J. S. BOWERBANK ON THE SPONGIADcE. 321 beneath, through the spaces between the apices of the conical organs. When the inner surface of this group of radiating organs is examined, the basal aperture of each is usually found to be completely open ; but in some cases, as represented in one of them at a, fig. 8, there is what appears to be tbe remains of a closing membrane ; it is, however, only in a very few cases that I have observed the remains of such an organ. These singular radial organs are not present on the distal end of the sponge for about two inches of its length ; and I examined this portion of it carefully in search of pores. I found the dermal membrane in a better state of preservation than on its proximal end, and its reticular structure well demonstrated. A few isolated pores in an open condition appeared at distant intervals; but many of these had more the aspect of orifices, by contraction of the membrane in drying, than of well-defined pores. I did not find any pores open in the pocilla beneath the radial coverings; but this is not surprising, as in dead sponges the general rule is to find them closed, and the exception is to find them open. The gemmules are not numerous. I found one only in the section at right angles to the surface of the pocillum represented by figure 7 ; but in other cups taken from close to the proximal end of the sponge there were considerable numbers of them disposed on the inner surface of the membrane, and on the outer surfaces of the sacculi of the disk there were a few attached to those surfaces ; but none were found within any of the conical sacculi, although their open mouths within the pocilla might have led us to expect to see some of them there. The gemmules are membranous, round or oval, semitransparent, and of a dark amber-colour, just such as we find in numerous other species of Isodictya. I searched in vain for them in the surrounding skeleton-tissues; but I found solitary ones only at distant intervals attached to the reticulations of more distant parts of the skeleton. Since the above description was written, on looking over a collection of sponges I purchased of the late Mr. James de Carl Sowerby many years since, I found another specimen of the species. It is similar in form to the figured one, is an inch shorter, and has neither its natural base nor its apex, but is rather larger in its diameter. Its inhalant and exhalant organs are exceedingly like those of the type specimen; but the latter are rather large. In their anatomical characters, the two specimens are in perfect accordance. DICTYOCYLINDRUS DENTATUS, Bowerbank. (Plate XXIX.) Sponge ramose, pedicelled, pedicel short and stout; branches very numerous, ascending, culminating towards the apex of the sponge, occasionally dividing or inosculating, furnished abundantly with stout tooth-shaped processes. Surface smooth. Oscula simple, minute. Pores inconspicuous. Dermal membrane coriaceous, profusely furnished with dentato-cylindro-hexradiate retentive and defensive spicula ; radii very short and stout; apices bi- or tridentate. Skeleton-spicula acuate, long and rather stout, and rarely cylin- PROC. ZOOL. Soc-1873, No. XXI. 21 |
