. The Annals and magazine of natural history; zoology, botany, and geology . hat of thestolon. Each of these now secretes a layer of silex, not, how-ever, with an intervening space between, but in the closest ap-position. In fig. 2 these two layers are indicated by the horse-shoe-shaped spaces numbered 1 and 2 respectively, but which,in all probability, are formed simultaneously. Now, as each layer becomes immediately consolidated and is * These cysts may readily be mounted in balsam; but, in the ordinarycondition of Amoeba it is impossible to preserve a vestige of outline whenso mounted. t A
. The Annals and magazine of natural history; zoology, botany, and geology . hat of thestolon. Each of these now secretes a layer of silex, not, how-ever, with an intervening space between, but in the closest ap-position. In fig. 2 these two layers are indicated by the horse-shoe-shaped spaces numbered 1 and 2 respectively, but which,in all probability, are formed simultaneously. Now, as each layer becomes immediately consolidated and is * These cysts may readily be mounted in balsam; but, in the ordinarycondition of Amoeba it is impossible to preserve a vestige of outline whenso mounted. t A vacuole may be defined as a space in a fluid of one density occupiedby fluid or solid matter of another density. Mineral Deposit in the Rhizopods and Sponges. 75 non-elastic*, no further deposit can possibly be effected fromeither surface unless by displacement of an equivalent volume ofsarcode, whether of the cavity or stolon. The next layer maytherefore be formed around the stolon as in layer 4, fig. 4, orby the retrocession of the boundary of the cavity, as in layer 3,. fig. 3. Most frequently the layers are deposited one within theother—this being the reason why the diameter of the first twolayers (which generally represent the diameter of the spicule) isnearly equal to the size of the external diameter, as statedby Dr. Bowerbank. But nevertheless successive layers of silexmay continue to be added externally as well as internally. Ifinternally, the stolon gradually diminishes in diameter, and mayultimately be altogether obliterated. Therefore, as a general rule,the greater the diameter of the tubular cavity of a sponge-spiculeand the thinner its wall, the younger is the spicule, and viceversa f. It has been stated that the first two layers secreted are inva-riably in contact. The proof of this is afforded by the fact thatspicules never occur in which there are two distinct hollowcylinders enclosed one within the other, and presenting a free * It is scarcely necessary, I p
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