The changes in shape and position of the sponge-cell are for the most part effected so imperceptibly that they may be likened to those which take place in a cloud. Its granules, however, are more active; but there appears to be no motion in any part of the cell (excepting among the molecules within the hyaline vesicle) which in any way approaches to that characteristic of the presence of cilia.

(57). The intercellular substance that forms the bond of union between the sponge-cells is of a mucilaginous appearance. When observed in the delicate pellicle which, with its imbedded cells, it forms over the surface and throughout the canals of the sponge, it is transparent; but when a portion of this pellicle is cut off from its attachments, it collapses and becomes semi-opake. In this state the detached portion immediately evinces a tendency to assume a spheroidal form; but whether the intercellular substance participates in this act or remains passive while the contraction is wholly performed by the habit of the cells imbedded in it to approximate themselves, is not evident.

(58). The freshwater sponges are reproduced from seed-like bodies found in the substance of the oldest or first-formed portions of the sponge, never in its periphery. They are round or ovoid according to the species, and each presents a single infundibular depression on its surface which communicates with the interior. At the earliest period of development at which it is recognizable, it is composed of a number of cells united together by an intercellular substance similar to that described above. In this state, apparently without any capsule, and about half the size of the fully developed seed-like body, it seems to lie free in a cavity formed by a condensation of the common structure of the sponge immediately surrounding it. The cells of which it is now composed appear to differ from those of the fully-developed sponge-cell only in being smaller, in the colourless state of their contained granules, and in the absence of hyaline vesicles. The seed-like body gradually passes from the state just mentioned into a more circumscribed form, then becomes surrounded by a soft, white, compressible capsule, which finally thickens, turns yellow, and developes upon its exterior a firm crust of silicious spicula, presenting in some species a hexagonally tessellated appearance (fig. 12, c.) The spicula are arranged perpendicularly to the surface of the capsule, and the interval between them is filled up with a white, sili-cious, amorphous matter which keeps them in position. Each spiculum extends a little beyond this matter, and supports on its free end a toothed disk, similar to a corresponding one on its fixed end, which rests on the capsule, so that the external surface of the seed-like body is studded with little stellate plates (fig. 12, d, e.) In other species, where there appears to be no such regular arrangement of these spicula, a number of smooth spiniferous points is presented.

Fig. 12. Magnified section of a seed-like body of Spongilla Meyeni, showing, f, spicular crust; g, coriaceous capsule; h, internal cells; i, infundibular opening, c, portion of coriaceous membrane, magnified, to show the hexagonal divisions with transparent centres; d, small spiculum, magnified; e, one of its toothed disks with central aperture, magnified. (After Mr. H. J. Carter).

(59). If a seed-like body which has arrived at maturity be placed in water, a white substance will after a few days be observed to have issued from its interior, through the infundibular depression on its surface (fig. 12, i), and to have glued it to the glass: if this be examined with the microscope, its circumference will be found to consist of a semitransparent material, the edge of which is notched or extended into digital or tentacular prolongations, precisely similar to those of the protean cell, which in progression or in polymorphism throws out parts of its substance in the same way. In the semitransparent substance may be observed hyaline vesicles of different sizes, contracting and dilating, as well as green granules, so grouped together as almost to enable the practised eye to distinguish in situ the passing forms of the cells to which they belong. Subsequently to the development of this fleshy substance comes that of the horny skeleton and its spicula (fig. 11, 2), which are at first membranous, and at an early period of their development pliable; they afterwards become firm and brittle. They are hollow, and the form of their cavity corresponds with their own shape; sometimes, moreover, they contain a green matter like the endochrome of the cells of Confervae*.

(60). In the genus Tethya, Mr. Huxley has described a true sexual generation to exist, - a portion of the spongy mass being found to consist of a granular substance in which ova and stellate crystalline bodies are imbedded. "The ova are of various sizes; they have a very distinct vitellary membrane, which contains an opake, coarsely granular yelk. A clear circular space, about 1/1600th of an inch in diameter, marking the position of the germinal vesicle, is seen in each ovum, and within this a vesicular germinal spot 1/5000 th of an inch in diameter is sometimes visible. The stellate bodies are about 1/2000th of an inch in diameter. The granular uniting substance is composed entirely of small circular cells about 1/3300th of an inch in diameter, and of spermatozoa in every stage of development from those cells. The cell throws out a long filament which becomes the tail of the spermatozoon, and, becoming longer and more pointed, itself forms the head. It is remarkable that the ova are in no way separated from the spermatozoa, but lie imbedded in the spermatic mass like eggs packed in sand*".

* Besides the seed-like bodies above described, other reproductive bodies are met with in Spongilla: - 1. some which, from their resemblance to the motile spores or zoospores of many plants, have also been termed swarming-spores (Schwarm-sporen); and 2. others which, from their resemblance to the spermatic filaments elsewhere met with, are denominated zoosperms.