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Show 8 DR. J. S. BOWERBANK ON THE SPONGIADcE. [J there remains scarcely a trace of the dermal membrane or of the sarcodous stratum immediately beneath it, and the greater portion of the sarcode of the interior is also destroyed. A few minute external defensive spicula were seen projecting from one portion when sectioned for examination, and a few small fusiformi-acerate ones lying parallel to the surface, apparently belonging to the membranous tissue, being all that I could detect. In some of the depressions of the surface of Dr. Fleming's specimen there are the remains of a very thin parasitic sponge with spinulate spicula, which might readily be mistaken for a portion of tbe absent dermal membrane, while in truth they have no organic connexion with the Geodia. The skeleton-spicula of this species, although similar in form to those of G. zetlandica, differ exceedingly in size, the latter having the average length of one eighth of an inch, while those of the former rarely exceed one eighteenth of an inch in length. They are slender in proportion, and intermingled with the fusiformi-acerate spicula ; there are a very considerable proportion of purely acerate ones exhibiting no tendency to the fusiform shape. The two forms of stellate spicula of the sarcode are indiscriminately mixed and dispersed in that substance; and the cylindro-stellate ones are very much the smaller of the two, requiring a linear power of at least 600 to render their forms visible; their extreme diameter does not exceed -J^VR inch, while the attenuato-stellate ones vary from J-JVO inch to -§\^ inch in diameter. The spicula of the membranes are few in number, small, and in many cases scarcely, if at all, fusiform. I did not observe any prolific or immature ovaria in the external crust of the sponge ; but in the internal membranes they were in every stage of development. The immature ones always appeared to be completely surrounded by a thick coat of sarcode; but this sarcodous coat was not apparent in the fully developed ones. The spicula figured are from Dr. Fleming's specimen, from the Island of Dominica. They are identical in all their structural characters with those from the specimens in the French Museum in my possession. GEODIA PERARMATUS, Bowerbank. (Plate II.) Sponge massive, sessile. Surface even. Oscula congregated in depressed areas. Pores inconspicuous, dispersed. Dermal membrane pellucid, spiculous ; tension-spicula fusiformi-acerate, minute, short, rather few in number; retentive spicula subsphero-stellate, radii attenuating, rather obtusely terminated, very few in number, and simple attenuato- or cylindro-stellate, very minute, abundant. Skeleton-fasciculi compact; spicula fusiformi-acerate, large and stout. Connecting spicula attenuato-patenti-ternate with bifurcating radii, very large and stout; and recurvo-ternate, small and slender, shafts long and attenuated ; also porrecto-ternate, minute and slender. Interstitial membranes-spicula the same as those of the dermal membrane. Ovaria spheroidal, slightly depressed. Colour in the dried state cream-yellow. |