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Show 1867.] DR. J. S. BOWERBANK ON HYALONEMA MIRABILE. 27 of the microscope, closely resemble a beautiful and elaborately constructed Gothic crypt. In this sponge the oscula are simple orifices, not projecting beyond the dermal membrane as in Hyalonema. But the same purpose prevails in both descriptions of cloacal organ, that of discharging the fsecal matters at a distance from the inhalant surface of the sponges. A section of one of the fajcal columns of Ciocalypta penicillus is represented of the natual size in the ' Philosophical Transactions of the Royal Society of London' for 1862, pl. 73. f. 4; and a magnified view of a portion of the same column is represented by fig. 5 ; and also in ' Monograph of British Spon-giadee,' vol. i. pl. 30. figs. 360 and 361. Elongated cloacal projections from sponges are by no means uncommon organs. In large specimens of Halichondria panicea and several other British species of sponges such organs are frequently put forth ; but in these cases the distal extremity is always open, and the production of these organs are the exception, not the rule : but the contrary is the case in the British genus Polymastia, very similar in its skeleton-structure to Alcyoncellum, Quoy et Gaimard (Euplectella, Owen, Trans. Zool. Soc. Lond. vol. iii. p. 203). In Polymastia mammillaris (Halichondria mammillaris, Johnston) there are frequently on a single specimen from forty to fifty of these cloacal organs, springing from a sponge about 2 inches in length and breadth and not \ inch in thickness, but attaining 1 inch in height, with a diameter of rarely more than 2 lines, the distal terminations being always closed; the minute oscula are dispersed on all parts of the cloaca, as in the corresponding organ in Hyalonema. Other British species of the same genus approach still closer to the form and peculiarity of LLyalonema. In Polymastia spinula the basal portion is exceedingly thin; and the cloacal projections, seldom exceeding two in number, are about an inch in length, being in height at least twenty times the length of the thickness of the basal sponge. In a third species of the same genus (P. bulbosa) we have a still closer approach in form to Hyalonema, the basal mass of the sponge being bulbous, in the form of a small onion, with a single long slender cloacal tubular appendage crowning its summit, with a length rather greater than the height of the bulbous mass beneath it (Monograph of British Sponges, vol. ii. p. 61). The structure of the column of Hyalonema, considered as a sponge, is not so anomalous as it at first appears. In truth it is only one of several varieties of such cloacal appendages, all of which approximate closely to each other in form. In Polymastia we have the cloacal organ hollow and closed at its apex, but supported by an external network of siliceous spicula, with the oscula dispersed over its surface. In Euplectella aspergillum, Owen, the skeleton is very similar to that of Polymastia, with the difference of the oscula being congregated at its distal extremity. In Ciocalypta the cloacal organs closely approximate to those of Hyalonema. Their parietes are thin, like those of Euplectella, Owen, with a central axis of spicula supporting the organ in an erect position ; in Hyalonema the spicula composing the column are exceed- |