The limits and object of the present article obviously forbid an extensive or very minute consideration of the anatomical details of each of these classes of animals, and we are com pelled to confine ourselves almost exclusively to such illustrations of their respective plans of organization as are afforded by the species referrible to each which inhabit the human body.
If a drop of the secretion of the testicle be expressed from the divided vas deferens in a recently killed mammiferous animal, which They appear to be formed in the seminal secretion under similar laws to those which pre side over the develop ment of other Entozna in the mucous secretion of the Intestines, &c., but are more constant in their ' existence, and must there fore be regarded as fulfil ling some more important office in the economy of the animal in which they exist.
They are not found in the seminal passages or glands until the full pe riod of puberty ; and in some cases would seem to be periodically deve loped. In the Hedgehog and Mole, which exhibit a periodical variation in the size of the testes in a well-marked degree, the Spermatozoa are not ob servable in those glands during their state of quiescence and partial atrophy. Professor Wag ner* examined the testes of different Passerine Birds in the winter sea son, when those bodies are much diminished in size. (See vol. i. p. 354, fig. 183.) They then con tained only granular sub stances, without a trace of the Spermatozoa. When the same bodies were ex amined in spring, they were found to contain spherical granules of dif ferent sizes and appear ances, (A, B, fig. 52,) which led to the suppo sition that they were the ova of the Spermatozoa in different stages of deve lopment, and capsules containing each a nume rous group of Sperma tozoa (C) were also pre sent ; whence it would appear that many of these animalcules were deve loped from a single ovum. In the semen contained in the vasa deferentia the Spermatozoa (D) were in great numbers, having escaped from their cap sules; they exhibit a re markable rotation on their has arrived at maturity, and be diluted with a little pure tepid water and placed in the field of a microscope, a swarm of minute beings resembling tadpoles will be observed moving about with various degrees of velo city, and in various directions, apparently by means of the inflexions of a filamentary caudal appendage. These are the seminal animalcules,
Zoosperms, or Spermatozoa (fig. 51): and, as it is still undetermined whether they are to be regarded as analogous to the moving filaments of the pollen of plants, or as independent or ganisms, it has been deemed more convenient to consider them zoographically in the present article as members of the class Entozoa.
The body to which the tail is attached is of an oval and flattened or compressed form, so that, when viewed sideways, the Zoosperm appears to be a moving filament like a minute Vibrio. It is this compressed form of the body which principally distinguishes the Sper tnatozoa or seminal Cercaria, from the true Ccrcarie of vegetable infusions, in which the body is ovoid or cylindrical ; the caudal ap pendage of the Spermatozoa is also propor tionally longer than in the Cercaria.
In some species of the latter genus an oral aperture and ocelliform specks of an opake red colour have been observed on the anterior part of the body, and they manifest their sen sibility to light by collecting towards the side of the vessel exposed to that influence. In the Zoosperms, which are developed exclu sively in the dark recesses of animal bodies, the simplest rudiments of a visual organ would be superfluous; they are, in fact, devoid of ocelli, and even an oral aperture has not yet been detected in these simplest and most mi nute of Entozoa. In neither the Zoosperms nor the Cerearia has the polygastric struc ture been determined. On the contrary, some of the non-parasitic species, as the Cercaria Lannte, are stated to have a true alimen tary canal, not polygastric.'* The Spermatozoa are not, however, the only examples of the present order of Protelinintha which have their habitat in the interior of living animals; many of the Entozoa themselves have been observed to be infested by internal para sites, which are referrible by their external form to the Cereariada.