This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(61). The multiplication of marine sponges, however, is effected in original, it attaches itself to a proper object, and, losing the now useless locomotive cilia, it becomes fixed and motionless, and developes within its substance the skeleton peculiar to its species, exhibiting by degrees the form of the individual from which it sprung. It is curious to observe the remarkable exception which sponges exhibit to the usual phenomena witnessed in the reproduction of animals, the object of which is evident, as the result is admirable. The parent sponge, deprived of all power of movement, would obviously be incapable of dispersing to a distance the numerous progeny that it furnishes; they must inevitably have accumulated in the immediate vicinity of their place of birth, without the possibility of their distribution to other localities. The seeds of vegetables, sometimes winged and plumed for the purpose, are blown about by the winds, or transported by various agencies to distant places; but in the present instance, the still waters in which sponges grow would not have served to transport their progeny elsewhere, and germs so soft and delicate could hardly be removed by other creatures.
Instead therefore of being helpless at their birth, the young sponges can, by means of their cilia, row themselves about at pleasure, and enjoy for a period powers of locomotion denied to their adult state.
* The Spermatozoa, until recently considered as animalcules, generally present themselves under the form of long slender filaments or corpuscles, the shape of which varies to a remarkable extent, and nevertheless is so constant in individuals belonging to the same species that it is frequently-possible to identify by their form the particular creature to which each modification is peculiar. Generally speaking, among the higher animals the Spermatozoa are found to consist of an extremely attenuated linear body, either filiform throughout or swollen and enlarged at one end, so as to present something like the appearance of a microscopic tadpole (fig. 13,1.) They are exceedingly minute, seldom exceeding a line in length, but much more generally of far smaller dimensions, so that the highest powers of the microscope are requisite for their examination. These microscopic atoms may be regarded not merely as abounding in the seminal secretion of all animals, but in fact as constituting that important agent, - the presence of a fluid or liquor seminis appearing, when regarded in a physiological point of view, merely the vehicle in which the active Spermatozoa are suspended.
Until very recently these minute bodies were regarded as individual animated creatures; and many authors have fancied that several forms of them at least presented a somewhat complicated organization, such as an intestine, gastric sacculi, and even generative organs1. More recent researches have, however, satisfactorily proved that they are in all cases composed of a uniform homogeneous substance of a yellowish colour, in which no traces of complexity of structure are discernible. Their movements, however, are in most cases exceedingly vivacious; and were it not for the now well-ascertained fact that many other constituent elementary tissues, both animal and vegetable, exhibit equal activity even long after their separation from the organisms to which they belong, we might still be tempted to assign to them a much higher position in the scale of vitality than that to which they are really entitled. The motions of the Spermatozoids are, however, evidently only comparable to the automatic movements of cilia, and the relationship which they bear another manner, which is the ordinary mode of their reproduction, and forms a very interesting portion of their history*. At certain seasons of the year, if a living sponge be cut to pieces, the channels in its interior are found to have their walls studded with yellowish gelatinous granules, developed in the parenchymatous tissue; these granules are the germs or gemmules from which a future race will spring; they seem to be formed indifferently in all parts of the mass, sprouting, as it were, from the albuminous crust that coats the skeleton, without the appearance of any organs specially appropriated to their development.
As they increase in size, they are found to project more and more into the canals ramifying through the sponge, and to be provided with an apparatus of locomotion of a description such as we shall frequently have occasion to mention. The gemmule assumes an ovoid form (fig. 10, B), and a large portion of its surface becomes covered with innumerable vibrating hairs, or cilia, as they are denominated; these are of inconceivable minuteness, yet individually capable of exercising rapid movements, whereby they produce currents in the surrounding fluid. As soon therefore as a gemmule is sufficiently mature, it becomes detached from the nidus where it was formed, and being whirled along by the issuing streams, is expelled through the fecal orifices of the parent, and escapes into the water around. Instead, however, of falling to the bottom, as so apparently helpless a particle of jelly might be expected to do, the ceaseless vibration of the cilia upon its surface propels it rapidly along, until, being removed to a considerable distance from its to ciliated epithelium-cells is rendered abundantly manifest by the revelations of the microscope to modern observers1. From these researches it would appear that the origin of the Spermatozoa is invariably to be traced to nucleated cells, in the interior of which they are individually developed. These developing-cells, or vesicles, as they are termed, are found at certain seasons crowding the seminiferous tubes of the testes in immense numbers.
Taken from the body after death they are seen to be perfectly transparent and filled with a fluid which on coagulating becomes somewhat granular. Most of these developing-cells (fig. 13, a, b, c) are found freely floating in the minute seminal canals, but frequently they are enclosed in another cell-like envelope, either singly (d) or in numbers of three, four, six, or seven in each; the existence, however, of a more considerable number (e, f) in one common cyst is unusual. Whether single or more numerous, however, it is in the developing-cells that the Spermatozoa are formed by a kind of endogenous growth, at first appearing like dim shadows lying amongst the contained granules, but gradually assuming a sharper outline as the body and, subsequently, the tail are perfected. The entire Spermatozoon at length becomes visible coiled up in the interior of the cell, which, when the development is completed, bursts and discharges its contents.