Although no distinct organs of generation have been detected, there is reason to suspect that the Spermatozoa are oviparous: they are also stated to propagate by spontaneous fission ; the separation taking place between the disc of the body and the caudal appendage; each of which develope the part required to form a perfect whole.
The Zoosperms of each genus of animals present differences of form or proportion, and frequently also differences of relative size as compared to the animal in which they are deve loped ; thus, in the figures subjoined, which are all magnified in the same degree, the Zoosperin from the Rabbit is nearly as large as that from the Bull, (jig. 51.) axis,which continues for five or ten minutes after the death of the bird in which they are developed. Some have supposed that these animalcules were the result of a putrefactive process, but this is disproved by their presence in testicles which have been removed from living animals, and by their ceasing in fact to exist when the seminal secretion begins to undergo a decom position. Their extraordinary number is such that a drop of semen appears as a moving mass, in which nothing can be distinguished until it has been diluted as befoie-mentioned, when the animalcules are seen to disengage them selves and commence their undulatory move ments. By means of the continual agitation thus produced the chemical elements of the fecundating fluid are probably kept in a due state of admixture. By the same movements the impregnating influence of the semen may be carried beyond the boundary which it reaches in the female organs from the expulsive actions of the coitus. It has been conjectured that from the rapid and extensive multiplication of these animalcules they may contribute to pro duce the stimulus of the rut. But the con sideration of the part which the Zoosperms may play in generation belongs to the Physio logical history of that function, and would lead to discussions foreign to the present article, which treats of their form and structure simply as the parasites of animal bodies.
In the human subject the form of the Zoo sperm is accurately represented in fig. 51.
Among the cold-blooded Reptiles the Zoo-. sperms of the , Frog (fig. St) have been ex amined with most attention, and have been the subject of interesting experiments in the hands of Spallanzani and Dumas.
The milt or developed testicle of the osseous Fishes abounds with moving bodies of a glo bular form. In the Shark and Ray the Zoo sperms are of a linear and spiral form.
The molluscous animals are favourable sub jects fur the examination of the present tribe of Entozoa on account of the great relative size of the parasites of the seminal secretion. They are mostly of a filamentary form, and have long been known in the Cephalopods. The Zoosperms of the Snail ( Helix Poniatia ) present an undulated capillary body, and move sufficiently Slowly to permit their being readily followed by the eye.
The Spermatozoa have been detected and described in the different classes of the Arti culate Animals. In Insects they are of a fine capillary form, and are generally aggregated in bundles. They abound in the semen of the Anellides and Cirripeds; lastly, these parasites have been found to exist in vast numbers in the spermatic tubes of the higher organized En tozoa themselves.
The second tribe of Protclmint/ia includes those cylindrical, filiform, eel-like, microscopic Animalcules which abound in decayed vege table paste, stale vinegar, &c. together with others which have attracted particular attention by the destructive waste caused by certain spe cies which are parasitic on living vegetables. These animalcules are termed Vibrionidee from their darting or quivering motion. They differ from the polygastric Infusories, not only in the absence of internal stomachs but also of external cilia, which is inferred by their not exciting any currents when placed in coloured water. They present a higher grade of organi zation than the Cercarian tribe in the presence of a straight alimentary canal, which is re markably distinct in some of the higher forms of the group, as the Gordioides and Ostia roides of Bory St. Vincent.
The higher organized Vibriones have distinct generative organs, and are ovo-viviparous.