The second example of an accessory digestive gland occurs in a species of Ascaris infesting the stomach of the Dugong: here a single elongated ccecum is developed from the in testine at a distance of half an inch from the mouth; and is continued upwards, lying by the side of the beginning of the intestine, with its blind extremity close to the mouth ; from the position where the secretion of this ccecum enters the intestine, it may be regarded as re presenting a rudimental liver.t Respiratory Organs.—The Entozoa have no distinct internal or external organs of respi ration. The skin in many of the Trematoda and Acanthacephala is highly vascular,I and the circulating fluids in these worms may be come oxygenated by contact with the vascular mucous membranes of the higher organized animals which they infest. In the Planarity the surrounding water is renewed upon the vascular surface of the body by means of the currents excited by the action of vibratile cilia; 'and the young of certain species of Distomata, which pass the first epoch of their existence under the form of Polygastric In fusoria, freely moving in water, are pro vided with superficial vibratile cilia arranged in longitudinal rows ; but these organs of lo comotion and adjuncts to the respiratory pro cess are lost when the Distomata resume their position as parasites in the intestines of the Fishes from which they were originally ex pelled.
Excretory glands.—As an example of an organ of excretion, we may refer to the glan dular sac lodged in the enlarged extremity of the Distama clavatum, which opens externally by a small orifice in the centrQ of that part,* and the corresponding cavities from which a clear or milky fluid is ejected by the posterior pores of some smaller species of Distomata and Diplostomata.f Organs of gencration.—The generative sys tem in the Entozoa presents great varieties in the form, structure, and combination of its several parts. Sometimes the female or productive organs alone are discernible. In many Ccstoidca, and in all the Tremotoda, the male gland is present and communicates with the oviduct, so that each individual is sufficient for itself in the reproductive capacity. In the Acanthocqhala and Nc matoidea the sexes are distinct, and a con currence of two individuals is required for impregnation.
No trace of a generative apparatus has hither to been detected in the Cystic Entozoa. They would seem to be gemmiparous, and to have the reproductive power diffused over the whole cyst, at least in the Acephalocysts, in which the young are not developed from any special organ, or limited to any particular part of the cyst.
The ovaries in the most simple of the Ccs told worms, as the Ligula, are situated in the centre of each joint, where they open by a transverse aperture, from which projects a small filamentary process or lemniscus, re garded by Rudolphi as a male organ. In the
Bothriocepholi the ovaries have a similar po sition, and in the Bothriocephalus talus (fig. 89) assume a stellated figure, with the aperture in the centre, which is situated in the mid dle of each joint. In the Both riocephalus microcephalas the ovary consists of one or two rounded corpuscles in the centre of the joints, but the generative orifices are margi nal and irregularly alternate, and the oviducts may be dis tinctly seen passing backwards to them.
In the Thnia Candelabra ria a sacciform ovary exists in each segment, which sends off an oviduct to the marginal outlet. Besides which, ac cording to Rudolphi, there is a longitudinal canal, uniting the different ovaries together, and undergoing a partial dila tation at the anterior part of each joint.—May not this be the male organ ? The androgynous structure of the generative apparatus is very well displayed in the Tape worm of this country, the Thula Solium.
In each joint of this worm there is a large branched ovariuin (i, fig. 90) from which a duct (h) is continued to the lateral open ing. The ova are crowded in the ovary ; and in those situated in the posterior segments of the body, they generally present a brownish colour, which renders the form of their recep tacle sufficiently conspicuous.* In segments which have been expelled separately, we have observed the ovary to be nearly empty, and it is in these that the male duct and gland is most easily perceived. For this purpose it is only necessary to place the segment between two slips of glass, and view it by means of a simple lens, magnifying from twenty to thirty diameters: a well-defined line (g), more slender and opake than the oviduct, may then be traced extending from the termination of the oviduct, at the lateral opening, to the middle of the joint, and inclining in a curved or • slightly wavy line to near the middle of the posterior margin of the segment, where it ter minates in a small oval vesicle. This, as seen by transmitted light, is sub-transparent in the centre and opaque at the circumference, indi cating its hollow or vesicular structure. The duet, or vas deferens, contains a grumous se cretion ; it is slightly dilated just before its termination.
In this species therefore, as also in Amphis tame conicum, the ova are impregnated in their passage outward. But in several species of Distomata, as D. clavigerum, OVilitt711, cirrige ruin, and in the Distama &Talcum, the ova escape by an aperture situated near the base of the penis, and reciprocal fecundation exists. The concourse of two individuals must also take place in those species of the genus Moms tomum, which, like the Itinnostomurn mutabile, are viviparous, and in which the orifices of the male and female parts are distinct.