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Show 1875.] SILICEO-FIBROUS SPONGES. 277 skeleton-structure appears to consist of two and sometimes three layers of this beautiful network. The outer and the inner layers are powerfully armed by a single series of stout harrow-like teeth projected from the angles of the network at right angles to the surface ; and each of these teeth is furnished with numerous imbricated scales, like the similar organs in Farrea occa, described in the ' Proceedings of the Zoological Society of London,' for M a y 13, 1869, p. 339. These organs in the species under consideration differ from those in F. occa in being longer and more slender in their proportions. If the specimen be examined by direct light with a power of 50 linear, these structures are exhibited in a very beautiful manner, projecting outwardly from the angles of the dermal layer of the quadrangular network at right angles to its surface, and from the internal skeleton-layer in like manner into the interior cavity of the sponge, while the intermediate layer or layers of the skeleton are entirely destitute of these organs; but in place of them they are abundantly furnished with attenuato-acuate entirely spined internal defensive spicula, which are based on the sides of the skeleton-fibres, and are projected at various angles into the areas of the rete. These internal defensive spicula are very numerous where the fibre is well coated with the dark amber-coloured sarcode; but when the sarcode is absent not a single spiculum is to be seen on the fibres. The skeleton-fibres of the interior surface do not appear to be furnished with internal defensive spicula; but those of the outer and intermediate layers are abundantly furnished with them. These spicula vary to a considerable extent in size, the longer being frequently twice tbe length of the shorter ones. The basal half of both is abundantly spinous, while the distal half is usually entirely spineless. The spines are short and acutely conical. I could not detect any interstitial membranes ; but a few large acerate spicula were observed in the interstitial spaces ; but whether they belonged to the sponge or were adventitious I could not determine, as several other forms of spicula were observed on both the outer and the inner surfaces of the sponge. O n examining the interior of the sponge with a lens of two inches focus through one of the large terminal orifices I observed a small patch of flocculent-looking sponge-tissue ; and on removing a portion of this and mounting it in Canada balsam, it proved to be a very young Hymedesmia johnsonia in an excellent state of preservation, with numerous specimens of that remarkable form of spiculum, the trenchant contort bihamate, dispersed among the acerate skeleton-spicula of that sponge, and closely resembling the portion of the type specimen represented in vol. i. plate xviii. fig. 293, ' Monograph of British Spongiadse.' The occurrence of this well characterized sponge parasitical within the Farrea is a remarkable circumstance, and it seems to indicate that the open tubular structure of that sponge is probably its normal condition. Beside Hymedesmia johnsonia, naturally parasitical on its inner surface, there were two other small groups of sponge-spicula that were evidently adventitious. One consisted of numerous short entirely spined cylindrical spicula distributed irregularly over the fibres of three or four areas of the skele- |