THE COMMERCIAL SPONGES AND THE SPONGE FISHERIES. 405 
The sponge of the markets is merely the skeleton, the supporting frame- 
work, which gives strength and form to the soft gelatinous tissues of the living 
animal. It is composed of a substance similar in general chemical and physical 
properties to silk, horn, and chitin, the basic material which forms the shells of 
insects and crabs. This material is distributed in a fibrous network, usually 
in accordance with a definite general pattern in each species; the diameters of 
the fibers, the sizes of the meshes, and the relations existing between the sev- 
eral parts lying within more or less well-fixed limits. In addition, the main 
fibers always contain more or less foreign matter, sand grains, spicules, etc., 
embedded in their substance in the form of a core. 
A casual examination of the living sponge will show it to be covered by a 
well-defined skin raised at more or less regular intervals into blunt little cones 
over the ends of the skeletal fibers, by which it is supported. Distributed 
over the surface, sometimes rather generally, sometimes locally, are sieve-like 
membranes, whose small pores lead into cavities lying just below the skin. 
From these cavities canals lead into the substance of the sponge, opening by 
numerous minute pores into as many small pear-shaped chambers, which from 
their opposite ends discharge through larger openings. If the canals leading 
from these could be followed, it would be found that, uniting with their fel- 
lows, they gradually increase in diameter until they open upon the surface of 
the sponge in one of the large conspicuous pores known as ‘‘oscula,” or, as the 
spongers call them, “eyes.’’ The oscula are sometimes more or less generally 
distributed, sometimes localized, according to the species, and each is sur- 
rounded by a smooth membrane capable of expanding or contracting in such 
manner as to vary the size of the opening. 
This canal system is one of the most important organs, as well as the most 
characteristic feature of the sponge. It is the sole means of feeding and prac- 
tically the sole means of respiration. Its method of functioning is as follows: 
The pear-shaped chambers described above are lined with cells of a peculiar 
character, collar cells, as they are called, each provided with a little lash or cilium 
projecting into the chamber and beating rhythmically in such manner as to set 
up a current in one direction. The mechanical effort of each is feeble, but the 
joint action of the untold numbers of such cells in a sponge sucks water through 
the small orifices in the surface, first described, into the ciliated chambers and in 
turn forces it into the successively larger canals until it finds vent through the 
oscula. The water, with its contained food and oxygen, therefore enters the 
sponge through the small superficial pores and leaves it by the large ones. 
Excluding from consideration the foreign bodies, shells, coral, etc., which the 
sponge often overgrows and surrounds, the whole interior, save the skeleton and 
spaces of the canal system, is occupied by tissues which are neither of many 
