The true nature of sponges has long been a matter of dispute, but they are now universally referred to the animal kingdom, their precise systematic position being still a matter of dispute.*
The Spongida may be defined as "sarcode-bodies destitute of a mouth, and united into a composite mass, which is traversed by canals opening on the surface, and is almost always supported by a framework of horny fibres, or of siliceous or calcareous spicula" (Allman).
From the above definition it will be seen that a sponge is composed essentially of two elements - a soft, gelatinous, investing "flesh," and an internal supporting framework or> "skeleton."
Taking an ordinary horny sponge as the type of the order, we find it to be composed of a skeleton (fig. 26) of horny reticulated fibres which interlace in every direction, and are pierced by numerous apertures, the whole surrounded externally and internally by a gelatinous glairy substance, like white-of-egg, the so-called "sponge-flesh." The horny skeleton is composed of a substance called "keratode," and is often strengthened by sand-grains, or by spicula of flint which also occur less abundantly in the sponge-flesh. These latter must not, however, be confounded with the skeleton of the typical siliceous sponges in which the keratode is wanting. Of the apertures which penetrate the substance of the sponge in every direction, some are large crateriform openings, and are termed "oscules," or "exhalant apertures;" whilst others, which occur in much greater numbers, are greatly smaller in size, and are termed "pores," or "inhalant apertures" (fig. 23). Both the oscula and pores can be closecd at the will of the animal; but the oscula are permanent apertures, whereas the pores are not constant, but can be formed afresh in the outer protoplasmic covering, whenever and wherever required. The "sponge-flesh," which invests the entire skeleton, is found upon a microscopical examination to be composed of an aggregation of rounded protoplasmic bodies - the so-called "sponge-particles" or "sarcoids" (fig. 24). Some of these are provided with a single flagellum, surrounded by a membranous collar (fig. 24, B); while others are capable of emitting pseudopodia from all parts of their surface (fig. 24, C). The former of these resemble Flagellate Infusoria, and the latter are similar to Amaebae and both possess a nucleus and contractile vesicle. Others of the sarcoids, again, become undistinguishably amalgamated with one another in progress of growth, and thus give rise to a so-called "syncytium," or layer of structureless sarcode. Regarding the skeleton as something superadded, we may therefore look upon a sponge as a kind of colony, composed of an aggregation of zooids, of which some are amoebiform, others are like Flagellate Infusorians, and others are specially modified to form a "syncytium." The first two kinds of these zooids are capable of procuring and assimilating food for themselves, and also of independent movements; and even fragments of the "syncytium," when detached, are capable of throwing out pseudopodia. This view of the true nature of a sponge becomes still more readily comprehensible when we consider the simplest condition in which a sponge occurs in nature (as exemplified, for instance, in certain of the Calci-spongiae, such as Sycandra, fig. 23, B); the condition, namely, in which the entire sponge consists, of a colony of sarcoids, secreting a common skeleton, but provided with only a single "oscu-lum," and a greater or less number of inhalant "pores."There are, in fact, many who hold that the more complex sponges are merely produced by the aggregation together of a number of these simpler colonies.
* High authorities consider the sponges as a division of the Caelenteraia, or as forming, under the name of Porifera, a division of the "Zoophyta" coequal with the Caelenterata. This view, however, is based upon the interpretation of sponge-structure adopted by Haeckel, and the sponges will be here regarded as referable to the sub-kingdom Protozoa.
Fig. 23. - A, Axinella polypoides, a fibrous Sponge showing oscula and pores; B, Sycandra ciliata, a calcareous Sponge, showing the single terminal osculum. (After Schmidt.)
Fig. 24. - A, Portion of Grantia, highly magnified, showing the triradiate spicules and the sarcoids ; B, A single sarcoid of Grantia compressa, greatly enlarged, showing the membranous collar (a), the flagellum (f), the contractile vesicles (c c), and the nucleus (n); C, A sarcoid of Grantia compressa, with the pseudopodia protruded and without the flagellum, greatly enlarged. (B and C are after Carter.)
The above-mentioned constituents of the soft parts of an ordinary sponge are usually disposed as follows: The simpler amoebiform sarcoids make up the bulk of the colony and form the greater part of the sponge-flesh. Embedded amongst these, however, in countless numbers, are spherical groups of flagellate sarcoids, those of each group so arranged as to enclose a central space, into which water is admitted in a manner to be subsequently alluded to. The sarcoids are so disposed that the flagella of all are directed inwards into the central space, and each has its flagellum surrounded by a membranous collar, within which the lash can be retracted when not in use (fig. 24, B). Each sarcoid can nourish itself, and can discharge the indigestible portions of its food; and the surrounding water is admitted into, or shut out from, the chamber formed by the sarcoids in accordance with the temporary needs of the colony. These spherical communities of flagellate sarcoids constitute the so-called "ciliated chambers," and are to be regarded as the essential elements of the sponge. Lastly, as we have seen, portions of the sponge-flesh cannot be resolved in this way into separate sarcoids, but the latter have apparently coalesced so as to form a continuous and seemingly structureless "syncytium." This change is especially liable to take place in the layer of sarcode ("dermal membrane " or "ectoderm") which covers the exterior of a living sponge.