The earliest appearance of this type is found in the cavitary Entozoa (Cwlelmintha), and the sexual organs, both in the male and female of these creatures, may be regarded as ex hibiting the greatest possible simplicity of structure, consisting merely of secreting tubes, which in one sex produce the seminal fluid, in the other develope the ova. The seminal organ, or testis of the male, is generally a single tube of extreme length and tenuity, winding in large folds around the alimentary canal, and occupying a large portion of the abdominal cavity ; when unravelled, its length is found to be many times that of the animal ; at one extremity it dwindles down to a filament of the utmost tenuity, which floats loosely in the juices of the body, whilst at the op posite end it terminates in a prolonged tubular penis, or organ of intrornission, placed near the anal orifice. In the females of some species, as in ilscariR, the oviparous system is composed of two tubes, each exceeding in length and tortuosity the seminal vessel of the male, and measuring in some cases upwards of six feet. These tubes, after becoming con siderably increased in size so as to form a kind of receptacle for the ova which they generate, unite prior to their termination in the vulva, the aperture of which is found upon the ventral surface of the body at about one third of its length from the anterior extremity. In Strougylus the ovarian tube is single, and its orifice nearer to the mouth. In many species, as Filaria, the young are produced alive, the ova being hatched in the oviduct, a sufficient proof of internal impregnation having been accomplished.
The Myriapoda, in every part of their struc ture, form the transition from the "land ida to the articulated classes properly so called. They are divided by entomologists into two classes, the 'aide or Chilognatha, and the Scolupen dride or Chilopoda, a division strictly in con formity with their internal structure; the former in fact represent the Annelida; like the Abran ciliate division of that class, they breathe by air-sacs, communicating with spiracles seen upon the exterior of their bodies. The Scolo pendra, on the contrary, respire by trachea, which permeate their viscera, as in the insect classes. In the generative system of these creatures a similar relationship is evident. In /a/us, the generative system occupies the ante rior segments of the body, the sexual apertures being found upon the rings near the cephalic extremity, whilst in Scolopendra they are placed, as in insects, near the anal orifice. As regards the internal sexual organs of lulus, but little is known conclusively, and our own researches upon this point have not been sufficiently satis factory to enable us to speak positively con cerning them, although the result leads us to suspect that in these creatures not only are the sexual parts analogous to those of the Anne lida, but that, as in many of that class, the ova are retained in cellular interstices surrounding the intestinal canal for some time prior to their expulsion.
In the Scolopendra the generative organs are more easily distinguishable, and much resem ble those of insects ; they ate, however, exceed ingly curious. In fig. 201 we have represented
the male apparatus of the Scolopcndra moral tans. The testes (a, a, a) are seven in number, and closely packed in parallel lines; each testis is composed of two parts, precisely similar to each other, which are seen separate at b; from each extremity of the fusiform testis arises a narrow duct, so that there are fourteen pairs of ducts arising from the fourteen secreting organs. Each of the testicular bodies is hollow inter nally. The ducts ultimately end in a common tube (c), which soon becomes enlarged and tortuous, terminating by a simple aperture near the anus. Just prior to its termination, the en larged canal receives live accessory glands, four of w hich (d, d, d, d) are intimately united, until unravelled, as seen in the figure, while the fifth (c) is a simple &wenn' of considerable length.
The ovarian system of the female Scolopen dra is a single tube, apparently without secon dary ramifications.
lnsccls.—In the numerous and diversified tribes of the insect world a great uniformity is observable in the general arrangement of the generative apparatus. The sexes are invariably separate, but while the internal organs are con stantly double and symmetrically disposed on both sides of the menial plane, the external parts which are subservient to copulation are removed to the posterior extremity of the body, and are single. Throughout the whole class the sexual system only arrives at that state of perfection which is compatible with reproduc tion in the perfect or imago state of the animal, although it may be detected in a rudimentary form even in the larva, being gradually more and more perfected during the developement of the pupa. The business of procreation in insects thus exclusively belonging to the per fectly formed creature, is accomplished only at the termination of their existence, and the whole tribe is remarkable from this circumstance.
The internal generative organs in male in sects are described as consisting of three por tions, the testes with their rasa deferentia, the vesiculm seininales, and the canalis excretorius. The testes, or, in other words, those portions of the apparatus which are supposed to furnish the essential part of the fecundatin4 fluid, like the rest of the glandular system, conzist of cam or utricles floating loosely in the abdominal cavity, immersed in the juices of the body, from which they derive their secretion. Never theless, although essentially constructed upon similar principles, the testicular caeca present a singular diversity of form in different genera, and some of the modifications are sufficiently curious, although in the present state of our knowledge it would be hopeless to attempt to explain the reason of their existence. Muller, from a comparison of the researches of various authors upon this subject, has given the follow ing summary of the principal forms of the sperm-secreting organs, and although the cata logue of varieties might doubtless be considera bly extended, those given will abundantly an swer our present purpose. Beginning from the tubular vessel, which is the simplest form of the testis, he traces it through the various com plications here enumerated.