Sponges present a point still somewhat problematical as to their true position in the scale of animal life, and there are still many doubtful or imperfectly known facts pertaining to their organization. As a family, sponges are very remarkable for the lack of a definite and constant form, have various habits of growth, are insensible to irritation and powerless to move about. The known species number now about three hundred, and new ones are constantly observed, in view of which fact it is obviously impossible to describe, within the limits of this sketch, more than the prominent characteristics of the type. The description following may be referred more particularly to the sponges of commerce.
Milne-Edwards and others have considered each sponge as one separate and distinct individual; but Prof. Clarke, Dr. Lockwood, Prof. Grant and others, regard them as an aggregation of minute infusoria, each within its living cell, and they are considered to begin life as solitary animalcules, and only in associations of these does the characteristic sponge structure appear. Of course this reduces it still lower in the plane of animal life, because individuality is soon lost when many of the same species are brought together and coalesce. The mass then may be considered in its complete state as being composed of three parts, namely, the skeleton, known, when cleansed, to commerce as the sponge; the fleshy matter, by which this is surrounded; and third, the spicules, or little spines, by which the fleshy matter is held in place.
The skeleton is a fibrous mass of complicated network of more or less regularity in the meshes, and of different patterns in the various, species. The fibre appears solid under the microscope, but in some species at least it contains within its interior another cylindrical thread, which usually becomes elegantly wavy or spiral under flexion, and probably adds to the elasticity. In some the skeleton is soft, compressible and very bibulous, from which there is a gradual passage into those of a rigid and compact texture, sometimes friable.
The first are composed of a horny substance, and the varying excess of silicious or calcareous matter causes it to lose more or less elasticity, and impairs its usefulness. As the skeleton is secreted by the fleshy mass of the animal, which is an active eliminator of the salt of the ocean, it is attempted to group them upon considerations of their building propensities, and the toilet and domestic sponges generally affecting horn, or keratose, are grouped as "keratora." With an excess of lime, as calcarea, building with silex—silicea. Various varieties exhibit all the colors, and many, are, while alive, very brilliant in appearance. Examining a toilet sponge, we find that the framework is covered with a membrane (not universal with all species), thin above, deep-brown on the sides and yellowish-brown towards the base. This investing membrane exhibits larger and smaller openings, is not spread smoothly, but appears irregularly covered with conical shaped points. Making a section of the still living sponge, we find the skeleton embedded in a glairy, gelatinous or albuminous substance, about the consistency of raw beef, with an exuding viscous, yellowish clear brown (in the bath sponge a clear gray) slime, called milk by Greek fishermen. This investing flesh has so little resemblance to ordinary animal tissue that it is technically called "sarcode." In this tissue we perceive a system of canals or pores, which run downward in all directions and enter directly certain little cavities or chambers connected with circuitous passages, which finally lead to large outlets or "oscula." The pores, belonging only to the flesh, are not visible in the skeleton, are very small, yet, compared with the cells, very large. In some orders of sponges the outer walls of the flesh open anywhere and everywhere for the admission of food, and no well-defined pores are visible, but open as required.
Prof. Grant first observed closely the ceaseless flow of liquid matter through and out of the living sponge. Another discussion has been had over the manner in which this is caused or maintained. Dutrochet, having made his celebrated discovery of that law of endosmose which regulates the transmission of fluids of unequal densities through organic membranes, was perhaps biased or prejudiced in applying the same law to the solution of this problem. At any rate, naturalists now agree that the flow is produced by the lashes or cilia, with which the unnumbered animalcules are each provided. The little chamber into which the pore opens has its wall lined with these uniciliated cells, and each lashes its cilium with vigor, and all harmoniously downward and inward, the effect vacuum above, the water, of course, passing in, being carried through the ramifications and out of the oscula with some vigor. Hurled along in the liquid are opaque masses, composed of excrementous particles, and at certain seasons ova and germicules, from which new beings are produced. We may consider the sponge mass as having a complete assimilating or digestive apparatus. The cells, or some of them, probably act as stomachal sacs, with the lashes for motors to attract food and repel refuse after the extraction of nourishment, which latter probably passes through the gelatinous mass into the general development. The sponge has such remarkable recuperative powers that one, being cut while alive, quickly rejoins, though not replaced in the same position. We show in Fig. 2 a group of living sponges.
Perhaps the most curious parts of the sponge structure are the spicula, or little spines, shown in Fig. 3. These, embedded in all parts of the sarcode, serve to bind the tissueless flesh in form. They serve as a natural felting, or as tiny anchors, generally composed of silex or lime, and are of an infinite variety of shapes, sometimes, occurring separately and sometimes in bundles, generally in this case bound together with horny matter. The spicules of the "sheep's wool" are of the latter class, and under a low magnifying power will be found generally broken and frayed. Spicules of silex are the most common and most variable in shape, and present every gradation, from simple needle to many-pointed star. Spicules of the same material, but of various forms, are found in the same sponge, but seem to occupy certain definite positions—some are peculiar to crust, some to sarcode and others to margins of canals, etc. They cannot be considered as formed by crystallization, as many could not so be produced; they exhibit more or less of a central cavity, in some of which organic matter has been discovered. Being beautiful microscopic objects, they have excited much attention, especially as they are the most lasting parts of the structure, and are distributed, not only over every known sea, being constantly dredged from the greatest depths, but they also in a large measure, in connection with other silicious bodies of like insignificance, go to form many large portions of the earth's surface. They occur in guano deposits, largely in the chalk formations, also in the tertiary formations in England, etc. Sponge tissue may often be observed in sections of agate, chalcedony, etc. Nodular flints are believed to be fossil sponges, as are also the celebrated "moss agates." Fossil sponges of many species are found in many localities in this country, notably in Kentucky and Tennessee. Spicules predominate so far in some sponges as to render them valueless, and their absence is always essential to its domestic uses.
The sponge is propagated by an ovum or germicule cast from the parent at certain seasons, this being usually a ciliated cell, which, floating for awhile, at last settles for life on some hard substance, and, giving no signs of sensibility, yet undergoes a change, the gelatinous flesh is riddled and channeled, the fibrous framework is formed, and the sponge is complete.
The process of generation is involved in some doubt, but they seem to contain within themselves the power, and seemingly by germination or budding. These germicules are apparently produced in some of the cells, and pass through the sarcode to the large canals, from which they are ejected, sometimes being protected and surrounded at their exit by spicules.
The duration of life and rapidity of growth are not known, though in the Mediterranean it is agreed that the ground may be fished over again in three years. The period probably varies greatly in different latitudes, for the species are widely distributed over the seas; they are scarce and small in cold latitudes, and increase most abundantly in the tropics.
Many strange and beautiful forms occur among sponges, some of which have received popular names, as Feather, Fan, Bell, Lyre, Trumpet, Distaff, Peacock Tail, Neptune's Glove, Neptune's Car, Venus' Flower-basket, Class Sponges, etc.
The cornucopia-shaped Glass sponge, the Venus' Flower-basket (Euplectella speciosa—the specially beautiful well-woven), is one of the rarest, and, as its name implies, one of the most beautiful varieties. It is found near the Philippine Islands. It resembles a structure of spun glass, and though apparently so delicate is yet quite strong; each thread, although of pure silex and seemingly solid, is really composed of a series of concentric tubes or cylinders, as if spun on a central thread. As the threads are of pure silex, one might suppose them to be perfectly transparent; but, on the contrary, they are translucent, and have a most exquisite opalescence. The structure has a woven fabric, and as it progresses it takes on the most quaint little flounces, with the most delicate frilled edges imaginable, and all arranged with such charming grace and ease. The lace-like structure is. so aerial a fabric, so quaintly graceful, and so deftly done in the putting together, that any embroidery would seem bungling in comparison.
The Cliona.— Sponges to which this name has been given have been observed to have the power of boring into substances the hardness of which might be considered a protection from such apparently contemptible foes. Shells, corals and solid rocks are broken up, and probably made available for the supply of the necessities of other creatures. The mechanism by which so low an animal produces so remarkable effects is still doubtful, but is attributed to a multitude of minute silicious crystalline particles adhering to the surface and set in motion by something analogous to ciliary action.
The Mediterranean and Red seas have from time immemorial afforded the finest sponges of commerce, and at the present time the most important sponge fisheries are those of the Grecian Archipelago and the coast of Syria, the products of which find their way to all parts of the world. The trade in sponges with Europe and America has of late greatly increased. Some three or four hundred boats are regularly engaged in the sponge fishery in these waters during the fishing season, which usually commences about the beginning of June and terminates at the close of October, the months of July and August being the most favorable for the pursuit of the industry. The method pursued by the sponge fisher is about as follows: A boat's crew of four or five men will scatter themselves along the coast for two or three miles, in search of sponges under the cliffs and ledges of rock. Those of inferior quality are found in shallow waters; the finer qualities are only found at a depth of from 20 to 30 fathoms. The first are fished for with three-pronged harpoons, by which they are forcibly torn from their rocky attachments, with more or less injury to their textures. The finer kinds are collected by divers, who descend to the ocean's bed and carefully cut them from their fastenings with a knife.
The sponge fisheries of the archipelago yield a large annual product, but the quality is not so fine as that of the Syrian fisheries, while the Syrian sponges in turn are surpassed in dimensions and fineness of tissue by the product of the Barbary coast. The sponge fishery of these favored regions is conducted without intelligent direction or thought of preserving the supply, in consequence of which the product, though it has of late largely increased, must in time become restricted in quantity from the failure of the supply. Well informed writers on this subject, in fact, assert that it is only a question of time when the trade shall altogether cease, unless some reform in the existing state of things shall be introduced, as the demand which every year clears the submarine fields of these sponges. causes such destruction that even the prodigious reproductive power which they possess is inadequate to keep up the supply.
To successfully counteract the rapid depletion of the sponge in these waters, it has been repeatedly proposed to naturalize the more valuable species on the French and Algerian coasts, and to protect their cultivation by special laws. It has been conclusively demonstrated that sponges may be artificially propagated with little trouble, and with successful results, from cuttings of the living sponge, so that no serious difficulty would be met with on this score; but thus far, for a variety of reasons, no practical measures of this kind have been taken.
The sponge fisheries in American waters are by no means insignificant, the principal localities of the industry being the Gulf of Mexico, the Florida Keys and the Bahama Banks. The quality of American sponges, however, as compared with those of the Mediterranean, is inferior. The annual product of the American fisheries is, nevertheless, very considerable. The fine Syrian sponge is distinguished by its lightness, its fine flaxen color, its fineness of texture, and its cup-shaped form. This sponge is specially employed for toilet use, and commands a high price. The fine sponge of the Grecian Archipelago is scarcely to be distinguished from that of Syria, though generally it is weightier, not so fine in texture, and the holes with which it is pierced are larger and fewer in number. The sponges of American waters are coarser in texture, harsher, and wanting in flexibility.—The Manufacturer and Builder, March, 1881.