44 INVERTEBRATE ANIMALS. 
for itself. As the sponge, however, is a fixed animal, some 
provision is necessary by which food shall be conveyed to the 
sarcoids in the interior of the mass. This is effected by a 
remarkable water-carrying or “ aquiferous” system in the follow- 
ing manner: The entire sponge is riddled in every direction by 
an immense number of canals, all opening on the surface, and 
communicating freely with one another in the ifterior of the 
mass. The canals are of different sizes, and, as can readily be 
observed in an ordinary sponge, their external Openings are also 
of different sizes. A few of the holes are of much larger size 
than the others, and these, for reasons which will be seen 
directly, are called the “exhalant apertures,” or “oscula.” The 
great majority of the holes are very minute, and these are known 
Fig. 1o.—Diagrammatic section of Spongilla (after Huxley). a@ a Outer or super- 
ficial layer of the sponge; 46 Inhalant apertures, or “‘pores;” ¢ ¢ Ciliated 
chambers ; @ An exhalant aperture, or ‘‘osculum.” The arrows indicate the 
direction of the currents. 
as the “inhalant apertures,” or “pores.” Ina living sponge a 
more or less constant circulation of water is carried on by 
means of this canal system. The water is admitted by means 
of the pores (fig. 10, 6), is driven into the interior of the 
sponge, and is finally expelled in steady streams from the oscula 
(fig. 10, Z). The mechanism by which this circulation of water 
is effected was long unknown, but it is now known to consist 
in aggregations of sponge-particles provided with cilia which 
all work towards the interior of the sponge (fig. 10,¢c). The 
circulation of water in this manner can be readily observed 
in many of our common marine sponges, and it is under the 
control of the animal to a great extent. The large apertures 
or oscula are permanent, but they can be closed and opened at 
will ; whilst the smaller apertures or pores appear to be formed 
