HUXLEY, ON CLASSIFICATION, 75 
plex a structure which results from the building up and massing 
together of a number of similar parts. 
The great majority of the sponges form a skeleton, which is com- 
posed of fibres of a horny texture, strengthened by needles, or 
spicula, of silicious, or of calcareous, matter; and this framework is 
so connected together as to form a kind of fibrous skeleton. This, 
however, is not the essential part of the animal, 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 ourselves with the 
skeleton, because that has nothing to do with what we are now 
considering—may be represented by the diagram (A, Fig. 3). 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 Ameba, 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 (b), 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 pro- 
longations 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 (b), which have thence been 
termed “inhalent.” ~ 
In the floor of the cavity, there are a number of apertures which 
lead into the canals ramifying in the deep layer, and eventually 
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, as 
Dr. Grant discovered, many years ago, strong, though minute, 
currents of water are constantly flowing out of these large apertures ; 
being fed by the currents which as constantly 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, 
remained for a long time unknown. It was rendered intelligible by 
