14 INTRODUCTION TO CLASSIFICATION. 



which is to be sought in that gelatinous substance, which invests 

 the fibres of the skeleton during life, and is traversed by canals 

 which open upon the surface of the sponge, directly or indirectly, 

 by many minute, and fewer large, apertures. 



If I may reduce a sponge to its simplest expression taking 

 the common Spongilla, for example, of our fresh waters, the 

 structure removing all complexities, and not troubling our- 

 selves with the skeleton, because that has nothing to do with 

 what we are now considering may be represented by the 

 diagram (A, Fig. 4). There is a thin superficial layer (a) 

 formed entirely of a number of the so-called sponge particles, or 

 ultimate components of the living substance of the sponge, each 

 of which is similar to an Amoeba, and contains a nucleus. These 

 are all conjoined in a single layer, so as to form a continuous 

 lamellar membrane, which constitutes the outer and superficial 

 layer of the body. Beneath this is a wide cavity, communicating 

 with the exterior by means of minute holes in the superficial 

 layer (&), and, of course, filled with water. The cavity separates 

 the superficial layer of the sponge from its deeper substance, 

 which is of the same character as the superficial layer, being 

 made up of a number of aggregated sponge particles, each of 

 which has a nucleus, and is competent to throw out numerous 

 pseudopodial prolongations if detached. While the living sponge 

 is contained in water, a great number of currents of water set in 

 to the wide cavity beneath a, a, through the minute apertures 

 (&), which have thence been termed " inhalent." 



In the floor of the cavity there are a number of apertures 

 which lead into canals ramifying in the deep layer, and eventu- 

 ally ending in the floors of certain comparatively lofty funnels, 

 or craters. The top of each of these presents one of those larger 

 and less numerous apertures, which have been referred to as 

 existing on the surface of the sponge, and which are fitly termed 

 " exhalent " apertures. For it has been discovered that strong, 

 though minute, currents of water are constantly flowing out of 

 these large apertures ; being fed by the currents which as con- 

 stantly set in, by the small apertures and through the superficial 

 cavity, into the canals of the deeper substance. The cause of 

 this very singular system of currents is the existence of vibratile 



