OCR Text |
Show 1875.] SILICEO-FIBROUS SPONGES. 563 structure of an irregular form, of an average diameter of half an inch. It is constructed of two, and in some parts of three, layers of rete, the intermediate layer, when present, having its fibres more slender and more irregularly disposed than the two external ones. The fibres of the outer structures are remarkably large and strong; an average-sized one measured ylg- in. in diameter; and the rete is more than usually close and compact; in many cases the areas do not exceed in breadth the diameter of the surrounding fibres. The form of the rete is mostly either square or oblong; aud its strength is greatly increased by the interior angles being replaced by curves, so that the areas are to a great extent either circular or oval. The external layer of tissue is sparingly spinous, and is also furnished with short and slender aculei, and abundantly with rectangulate sexradiate defensive organs, based most frequently on the sides of the fibres and projected thence into the areas of the network; and the aculei are apparently the nascent state of these organs. There are strong appearances of the specimen having been part of a cup-shaped sponge ; at one portion of it the skeleton-rete is closely and irregularly as it were crushed together; from this part the primary skeleton-fibres radiate in a fan-shaped mode, the secondary ones assume the state of a series of concentric curves, and the reticulation increases in regularity of structure as it approaches what has evidently been the distal portion of the cup-shaped structure ; and here it is that we find the dermal membrane and its characteristic spicula in the finest state of preservation. The dermal membrane is extremely pellucid, and would scarcely be visible if it were not for the numerous retentive spicula adherent to its surface. The bihamate retentive spicula are numerous, uniform in size, and very slender; their curves are about three fourths of a circle; and they are equably dispersed over the surface of the membrane. They do not exceed 8^/inch in length. The minute bidentate anchorate ones are very few in number; one of the largest of them measured l0\0 inch in length : they vary in their mode of development to some extent; and occasionally there is a third, small tooth, more or less produced, between the two large lateral ones. The attenuato-subspi-nulate internal defensive spicula are also very numerous; they vary somewhat in size, and are entirely incipiently spinous; one of the largest measured y ^ inch in length ; they are based on the inner surface of the dermal membrane, and are projected inwards at various angles, while others are seated on the sides of the dermal skeleton-rete, and are projected into the areas at various angles. The whole three forms abound, not only on those parts of the membrane covering the areas of the reticulations of the fibres, but they also occur on the parts attached to the skeleton-fibres, so that no part of the dermal membrane is left unprotected. The central canals of the skeleton-fibres are small ; they vary to a slight extent iu some parts of the rete, and in some of the largest of the fibres they are entirely obsolete. The rectangulate sexradiate organs are not very numerous near |