Although from the relations of the 111ollus cons division of the animal kingdom we might infer that a more elevated type of structure would characterize their organs of reproduction, the present state of our knowledge of the anatomy of these creatures compels us to arrange the lowest orders of that extensive class with those tribes which only possess an ovigerous system ; for although an androgynous confor mation is presumed by many to exist in all Bivalves, the presence of any superadded im pregnating portion has not yet been pointed out, and even the course of the ova in their passage from the ovarian cavity remains a mat ter of speculation. In the Conchiferous order, from causes sufficiently obvious when we con sider the peculiar structure of the animals which compose it, the full development of their numerous ova could not be accom plished in the ovary itself, which occupies a large portion of the body, as any material in crease of bulk produced from this cause would materially interfere with the closing of the shell ; at an early period, therefore, the ova are transferred from the nidus in which they were formed to the bronchial fringes, between the laminae of which they perfect their growth, and are fully exposed to the influence of the element around them. Oken traced a canal through which he supposed the ova to be con veyed directly from the ovaria to the gills;1 but notwithstanding his observations Carus contends I that the eggs pass into the stomach through one of the openings hitherto considered as belonging exclusively to the biliary ducts, whence they are evacuated through the mouth and conveyed into the openings of the gills by the water which flows between the pallial laminar from before backwards, and ultimately escape by two canals which•open below the anal tubes.
In the Tunicata, and also in those forms of the Gasteropodous Mollusca which most nearly approximate the Conchifera in the details of their organization, the ovary is imbedded in the substance of the liver, and the ova are dis charged through a simple duct, unprovided with any appendage which can be looked upon as a male apparatus. It is true, indeed, that in all these cases the walls of the oviduct may themselves furnish a fertilizing fluid, and by many physiologists they are supposed thus to supply the want of male parts; such an hypo thesis, however, is, to say the least of it, entirely gratuitous ; but as it is more our busi ness to trace the development of organs than the modes in which their deficiency may be supplied, we are content to leave the question without further discussion in this place.
Animals provided with ovigerous organs combined with an addi tional secreting structure, probably sub servient to thefertilization of the ova.
In this type of the generative system it must be obvious that the function attributed to the superadded portion is by no means indubitably substantiated, the opinions of physiologists relating to its office being rather based upon analogical reasoning than supported by direct evidence; and, in fact, some authors deny entirely that a necessity for the impregnation of the ova is more evident in this division than in the last. Nevertheless, although it is im
possible distinctly to prove the identity in function between the appended portion and the testis of higher forms of organization, the evidence afforded from the position which it invariably occupies, and from the considera tion of the parts connected with generation in diceeious animals to which we are insensibly conducted by this species of Hermaphrodism, is sufficiently cogent to warrant our application of the term ovarium to the nidus wherein the ova are produced, and to justify us in designa ting the accessory organ as a testis or apparatus for impregnation.
The Twnioid Sterelmintha furnish us with one of the simplest examples of this arrange ment of the generative organs. In the long and tape-like bodies of these Entozoa each segment, with the exception of the smaller ones near the head, possesses distinct ovigerous and impregnating structures. The female part of the apparatus occupies the centre of the joint, and consists of lateral tubes ramifying from a central canal, which at times may be seen to be full of minute granular ova. From these ovigerous canals a duct issues, which commu nicates with the lateral pore and receives before its termination two delicate tubes, recognizable under the microscope as dark lines imbedded in the pulpy segment, and which may be pre sumed to furnish an impregnating secretion.
In the RotifCra, or wheel-animalcules, the female apparatus consists of two long and comparatively wide sacculi, in which the ova are developed ; these open at the anal orifice, and receive near this point two narrow cceca, which, as in the last case, may secrete a fer tilizing fluid, serving to impregnate the eggs prior to their expulsion. The ova of these minute creatures, before the escape of the young, are exceedingly beautiful subjects for the microscope, the wheels of the embryo being easily distinguished in rapid action through the pellucid coverings of the egg.
In the Cirrhopoda we have most probably an example of this mode of generation, pre suming, that is, that the opinions of Cuvier upon this subject are correct. These opinions, it is true, have been disputed by various authorities, as will be evident on reference to the article CI 1111110PODA ; but their correct ness has been so fully supported by the dissections of John Limiter, recently given to the world,* that it seems best at least to pause before repudiating the conclusions to which these great anatomists, unacquainted with the labours of each other, were indivi dually conducted. In the Cirripeds the ovaria are two in number, placed on each side of the stomach ; the two oviducts which proceed from these unite to form a single elongated tube, the parietes of which are thick and apparently glandular. It is evident that in this case the walls of the common canal, or ovipo sitor as it is usually termed, may serve to secrete a seminal fluid, impregnating the eggs at the period of their extrusion; and such, in the opinion of the authors above mentioned, is a part of its office.