736 MARINE PRODUCTS OF COMMERCE 



it is not unlike beef liver, and is traversed by numerous chambers and canals. 



It is covered by a skin raised at intervals into blunt little cones and perforated 

 by small pores leading into subsurface cavities. These canals run into the interior, 

 their walls punctured by innumerable minute holes, each of which leads into a 

 microscopic pear-shaped chamber, which in turn opens by a larger pore into a 

 canal. These canals, uniting with others, become gradually larger until they again 

 reach the surface in one of the large openings scientifically designated as oscula, 

 but known to the spongers as "eyes" or "vents." 



The pear-shaped chambers are lined with innumerable microscopic lashes or 

 cilia, which, by their rhythmic beating, set up a current of water entering by the 

 small pores and escaping from the vents, thus carrying food and oxygen to all 

 parts of the sponge. 



There are tissues of various, but not markedly differentiated, kinds in the fleshy 

 mass of the sponge; but the important economic feature is the skeleton, which, 

 freed from the fleshy matter, constitutes the sponge of commerce. This is com- 

 posed of a substance related in general chemical and physical properties to silk, 

 horn, and chitin, the foundation of the shells of insects and crabs. This substance, 

 called spongin, is arranged in a fibrous network varying in structural details in the 

 different sponges; but for each the thickness of the fibers, the sizes of the micro- 

 scopic meshes, and the relations of the several parts, one to another, lie within 

 definite limits of variation. Embedded in the core of the large main fibers there 

 is always some foreign matter, such as sand grains; but an excess of such materials 

 in or on the fibers makes the sponge harsh and otherwise inferior for commercial 

 purposes. 



Little is known of the life history of the commercial sponges. In some species, 

 if not all, the sexes are distinct, females preponderating. The young produced 

 from the eggs are free-swimming organisms, and are still very small when they 

 settle and become permanently attached. This must be a critical stage in their 

 life, for they are so minute that a very thin stratum of silt would be sufficient to 

 engulf and smother them. As much of the sea bottom is covered with soft or shift- 

 ing deposits, the mortality at this period must be very high. 



Within the limits of variation of the species sponges, in shape, rate of growth, 

 texture, etc., are creatures of their environment, and when transplanted are often 

 quickly modified in general characters. They require for their existence water of 

 nearly full oceanic salinity. If kept moist they tolerate exposure to the air for 

 considerable periods during cool weather, and in Florida they grow from about 

 low-water mark to a known depth of 150 feet, and probably more. In the Medi- 

 terranean they occur to depths of 500 to 600 feet. That the food is taken into 

 the sponge wholly or in large part through the canal system is practically certain, 

 but of what it consists is not known. 



Description of Commercial Sponges 



The sponges of commerce, as they appear on the market, are only the skeletons 

 or supporting framework from which the soft fleshy matter of the living animal 

 has been removed. This skeleton consists of fibers of a material which has been 

 called spongin, and it is upon the physical properties of this material and the 

 way in which the fibers are disposed that the economic value of sponges depends. 



The fibers of commercial sponges vary in diameter, but are always fine and 



