Where Do Sponges Come From?
Until within comparatively recent years, the sponge was regarded as a plant; it is now known to belong to the animal kingdom, and to the order spongida of the class of rhizopoda. Sponge is an elastic, porous substance, formed of interlaced horny fibers, which produce by their numerous inosculations, a rude sort of network, with meshes or pores of unequal sizes, and usually of a square or angulated shape. Besides these pores there are some circular holes of large size scattered over the surface of most sponges, which lead into sinuous canals that permeate their interior in every direction. The oscula, canals, and pores, communicate freely together. The characteristic property of the sponge is the facility with which it absorbs a large quantity of any fluid, more especially of water, which is retained amid the meshes until forced out again by a sufficient degree of compression, when the sponge returns to its former bulk. From this peculiarity, combined with its pleasant softness, arises the value of the sponge for the purposes to which it is applied. In domestic economy and in surgical practice, there is no other product that can be satisfactorily substituted for it.
Sponge is an aquatic production, indigenous to almost every sea and shore. It is abundant and varied between the tropics, but becomes less so in temperate latitudes and continues to diminish in quantity, variety, and size, as it is traced into European and colder seas, until it almost disappears in the vicinity of the polar circles. Some sponges are known to be hermaphrodite, but that the individual at one period produces chiefly male elements, and later, chiefly female elements. Fertilization takes place in the body of the mother, and the egg here undergoes its early development. The embryo eventually bursts the maternal tissue and, passing into one of the canals, is caught by the current sweeping through the canal system and is discharged into the surrounding water through one of the large apertures on the surface of the sponge. In the Bahama Islands and along the coast of Florida, the breeding time of many sponges covers the period from mid-summer on through early Autumn.
There is propagation sometimes by ciliated gemmules, yellowish and oval, arising from the sarcode mass, and carried out by the currents. These are mostly formed in the spring, and after swimming freely about for a time, become fixed and grow. In its natural state, the sponge is a very different looking object from the article of commerce. The entire surface is covered with a thin, slimy skin, usually of a dark color, and perforated to correspond with the apertures of the canals. The sponge of commerce is in reality only the home or the skeleton of the sponge.
There are a few sponges that inhabit ponds and sluggish rivers; the others are marine. Of these, many of the calcareous and siliceous kinds inhabit the shores between tide-marks, preferring a site near the low ebb, where, nevertheless, they are daily alternately submerged, and left exposed to the atmosphere. The figured sponges with a fibrous texture, to whatever genus they belong, are denizens of deeper water, and are never left uncovered. They grow usually in groups, on rock shells, shellfish, corallines, and seaweeds, and either have no power of selection, or the quality of the site is indifferent to them.