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culating over the little sarcoid for its aeration and nutrition; and, lastly, that each little sarcoid has an independent vitality which enables it to live when separated from the parent mass, and under favourable circumstances to become the commencement of fresh sponge growths. Also the living sponge presents an aquiferous system, consisting of a series of anastomosing channels passing through its mass, through which currents of water are constantly passing, entering by the minuter orifices or pores, and escaping from the larger openings or oscula, in this way bearing floating nutritive material to the fixed sarcode matter and at the same time aerating the whole animal. Such are the general characteristics of living sponge; the skeleton which we see in dried specimens is, in the ordinary sponge of every day life, composed of horny material in form of tubes, in composition resembling animal matter, and of a softness and elasticity which gives it its domestic value. These sponges are called keratose or horny, and form one of the three groups into which sponges are divided by the nature of their skeleton. In a second order the framework is calcareous, composed of carbonate of lime, and there is yet a third to which Hyalonema belongs in which the skeleton is silicious. Whatever may be the composition of the fibres forming the framework, they are arranged always in an intricate network, thus securing the peculiar cellular structure which sponges possess. For the purpose of strengthening the skeleton and assisting in giving support to the semi-diffluent sarcode matter, we also find, especially in the calcareous and silicious sponges, what are termed spicules, that is, calcareous or silicious particles of definite form, generally slender, acicular, sometimes needle-like in shape, in other cases assuming very graceful and elegant forms. The Hyalonema is particularly rich in these, but as we shall have to return to them in speaking of the polythoa or bark, it will be convenient to consider them then.
So far we have met with little in the general formation of Hyalonema different from siliceous sponges generally, nothing indeed, except the variety of beauty of the spicules
