—Ovigcrous and impreg nating organs co-existent, but the co operation if two individuals necessary for mutual impregnation.
This arrangement of the generative system occurs in some of the Parenchymatous Entozoa, in the Annelida, and also in the Pteropod and some Gasteropod Mollusca. Some of the Entozoa, as Fasciola and Planaria, furnish the simplest examples of this hermaphrodite condition. In these creatures the male organs consist of spermatic caeca, communicating with a minute extensible penis, which is placed behind the oral sucker. Near the penis a small orifice is seen, leading to the ovigerous canals, which have no communication with the impregnating apparatus; and the copula tion of two individuals is thus indispensable to a reciprocal fertilization of the ova.
Those of the Annelida in which the gene rative system is best understood are androgy nous, and mutually impregnate each other, although it is probable that in the tubicolous genera, which are immoveably fixed to the same spot, and almost deprived of locomotion, each individual may in itself be sufficient for reproduction.
In the Abranchiate and Dorsibranchiate Annelida the male apparatus is composed of several pairs of secreting bodies, arranged on each side of the menial plane, those of the same side communicating with each other by a common vas deferens. In the Leech the visa dcferentia, which convey the secretion of the numerous testicular masses, terminate in a long protractile tubular penis, and at a short distance behind this the opening which leads to the female parts may be discovered. These latter consist of a simple uterine sacculus, or receptacle for the ova, to which two minute ovaries are appended. The congress of two individuals is effected by the reciprocal in troduction of the organs of intromission into the vulva;. In the Earthworm and Nais the intromittent apparatus is deficient, so that some authors have even doubted that the process of copulation, which is undeniably essential to fecundity, does more than stimulate each individual to self-impregnation. In the Earth-worm, as well as in Arenicola and Aphrodita, the ova, after escaping from the ovaria, are retained in the cellular meshes which surround the alimentary canal, in which they are not unfrequently hatched, the young being most probably expelled through a tubular aperture at the posterior extremity of the body.
As regards the generative system, the Pte ropod Mollusca approximate the more complex type seen in the Gasteropoda. In Clio borealis, the ovary, which is partially enveloped by the liver, gives off a slender duet, which, after a short course, plunges into a glandular tube ; this, becoming gradually narrower, terminates in a round sac placed on the left side of the head, where it opens externally : near this point is the penis, or organ of intromission, communicating with a small sacculus, by which the male secretion is probably furnished.
The most complicated forms of this species of hennaphrodism are met with in the Gas teropod division of Mollusca, existing through out the Nudibranchiate, Tectibmnehiate, In fcrobraneliiate, and Heteropod orders, as well as in those pulmonary genera which are un provided with a calcareous operculum. In all these cases the testis is single and divided into lobuli, connected together by the divisions of the vas deferens so as to exhibit a racemose arrangement, and each lobule, on minute in spection, is found to consist of little peduncu lated vesicles (fig. 200). A slender vas defe rens conducts the secretion of this testicle to the base of an intromittent organ of a most sin gular description ; this is a muscular tube of great length, which, when not in use, is in verted and concealed within the body, but ca pable of protrusion at the will of the animal. The female portion of this system is composed of one ovary, provided with an ample and tortuous oviduct, which serves, indeed, as a kind of uterus or egg receptacle, wherein the ova are retained until ripe for extrusion. Near the termination of this oviduct are placed several additional appendages, some of which are apparently destined to furnish an invest ment for the ova, whilst one, which is con stantly present, is probably a reservoir for the seminal fluid required to fertilize the eggs as they arc expelled. (See GASTEROPODA.) The external parts are so disposed that during the copulation of two individuals the male organ of each is introduced into the orifice leading to the female apparatus of the other, both thus impregnating and being im pregnated at the same time.