ON THE SKELETONS OF ZOOPHYTES. 
175 
are the most inferior group. They abound in the seas of our northern latitudes, 
and have their Spongy skeleton strengthened with Silicious needle-shaped spiculse, 
embedded with exquisite regularity in a glutinous material, to compose fasciculi, 
which are interwoven into different forms to constitute a network, that traverses 
the organism, bounds the aquiferous canals, and supports the gelatinous or¬ 
ganised tissue of the animal. The form of the spiculse varies in the different 
genera; and these, being permanent, afford the best characters for distinguishing 
the same. 
In the second order the skeleton is consolidated with Calcareous spiculse, 
which give a whitish colour to the beings comprised in this group; hence they 
are called I^euconida (Xevxos, white). They assume a more complex form than 
the Halanice , and their skeleton is extremely friable, from the difference in the 
density of its mineral ingredients. Grantia compressa of our coasts is typical of 
this order; the surface of its white ovate sac-like body is perforated with minute 
holes for imbibing the water; these are protected with Calcareous spiculee of 
different forms; externally they are thick, and bent at one extremity, tapering 
gradually to a sharp point at the other, whilst the larger inner surface of the pores 
is surrounded with spiculse of a tri-radiate form. In Grantia ciliata the surface is 
closely covered with linear pointed spiculse, whilst in its substance are embedded 
others of a tri-radiate form. The pores, thus carefully guarded, are the mouths 
of afferent canals that ramify through the body, and open into eferent channels 
or vents, through which the fluid that has circulated in the animal is ejected again 
into the sea. 
The members of the third order, Keratosa (Ke^m, a horn), have their skeletons 
composed of horny tubular filaments interwoven into a network, and disposed 
around the pores, and the afferent and eferent canals. The Common Sponge 
(Spongia officinalis) is typical of this order, and affords an interesting example 
of the complicated mechanism displayed in the skeletons of this class. The 
innumerable small pores that crowd the surface of this zoophyte lead into tortuous 
canals that wind and anastomose together throughout the structure of the Sponge, 
until they open into other channels terminating in the large orifices or vent 
through which the inhaled water that has circulated in its porous texture issues 
again into the sea. The Keratosa are confined to the warmer regions of the ocean, 
and the numerous species which this order comprehends attain considerable mag¬ 
nitude. When growing from rocks at the bottom of the shallow translucent 
Indian seas, they present the appearance of large shrubs adorned with the most 
beautiful colours. The albuminous material constituting the base of the skeletons 
of these animals, whether strengthened or not by the deposition of earthy matter, 
is a secretion due to the vital energies of the enveloping organised gelatinous 
substance; and like the arborescent stems of Volypifera , or the shells of Mollusca t 
