RHIZOPODA. , 45 
afresh, wherever they are wanted, at any point of the surface. 
By means of the currents of water each individual sarcoid or 
sponge-particle is.enabled ‘to obtain food, so that the whole 
sponge, as remarked by Huxley, “represents a kind of sub- 
aqueous city, where the people are arranged about the streets 
and roads in such a manner that each can easily appropriate 
his food from the water as it passes along.” It is also not 
improbable that the process is at the same time a rudimentary 
form of respiration. 
Such, then, are the general phenomena exhibited by any 
sponge, and the chief variations which occur amongst the 
sponges are to be found in the nature of the skeleton. In the 
Fig. 1r.—a Gemmule of Sfongilla; h Hilum; 4 Diagrammatic section of the 
gemmule, showing the outer layer of spicules or amphidiscs, and the inner mass 
of cells; Z Hilum; c One of the amphidiscs seen in profile ; d Fragment of the 
skeleton of a horny sponge (after Bowerbank), showing the interlacing horny 
fibres with spicula. All much magnified. 
sponges of commerce the skeleton consists of matted fibres 
composed of a substance nearly allied to horn. In other forms 
the skeleton is calcareous, or composed of lime; and in other 
cases, again, it is siliceous, or composed of flint. The Venus’s 
flower-basket, which looks like a goblet woven of spun glass, is 
a familiar example of the flinty sponges. In most cases the 
skeleton, and often the flesh as well, is furnished with more or 
less numerous needles or spicula, generally of flint, but some- 
times of lime, which assume a great variety of shapes, and 
appear to exercise different functions (fig. 11, ¢, @). 
As regards the reproductive process in the sponges, it will be 
sufficient to state very briefly the leading phenomena which 
