In the genus Diplostomum, in which the nutritious and vascular systems characteristic of the Trematoda are peculiarly well displayed, (fig. 81,) a short and slightly dilated canal is continued from the mouth, and soon divides into two alimentary passages or intestines, e, e, which diverge, and proceed in a slightly un dulating course, towards the hinder sacciforin appendage of the body, dilating as they de scend, and ultimately terminating each in a blind extremity, f,f. The contents of this long bifid blind alimentary canal are of a yellowish brown colour, especially in old individuals, and consist of a finely granular substance. As there is no separate anal aperture, the crude and effete particles are probably regurgitated and cast out by the mouth, as in all other Trematoda.
The posterior projection of the body, g, Nordmann compares to the posterior appen dage in the Cercariic ; it is terminated by a posterior aperture which seems to be the ex cretory outlet of some secerning organ ; since a milky fluid is sometimes ejected from it with force. In a species of Distoma (Distoma clavatum, Rud.) which I recently dissected, there is a similar aperture which forms the outlet of a vertically compressed sac ated between the cliyie-receptaeles (see actions of the Zoological Society, plate 4, p. 381, pl. 41,figs. 17, 18, d, g). In the Diplostomum volvens Nordmann supposes the aperture in question, h, to be the termination of a canal continued from the oviduct. Besides this canal the posterior appendage of the body is occupied by a sac of a corresponding form containing a milky fluid, i, i, and to which the term of chyle-receptacle is given by mann, as was previously done by Laurer to a corresponding cavity in the Amphistoma cum. The nutritious contents of this canal would seem to exude through the parietes of the coeval extremities of the intestines, as no distinct aperture of communication is obvious. Two vessels, k, k, are continued on each side from the anterior and external part of the chyle receptacle; they extend forwards to the anterior third of the body, and are there brought into munication by a transverse vessel, I, I, which tends across the dorsal aspect of the body. From the point of union of the transverse with the external lateral vcssels,a vessel is continued ward on each side, appearing as the continuation of the external lateral one. These vessels, In, rn, arc reflected inward at the anterior angles of the body, and unite in the middle line to form the vessel, n, which may be regarded, according to Nordmann, as representing the arterial trunk, and which is continued to the posterior extremity of the body, distributing branches on each side throughout its whole length.
mann observed a circulation of fluid in the vessels marked m, m, which was panied by any pulsation, and which may fore be compared to the cyclosis of the nutrient fluids in the vessels of trica, Polypi, and other Acrita, and is probably due to the action of vibratile • cilia.
In a few species of Pla narix the mouth is terminal and anterior, as in the Distornala ; these form the subgenus Prostorna of Professor Duges.* In the greater number of these non parasitic Sterelmintha the al imentarycanal commences from a cavity situated at the middle of the inferior sur face of the body. A pro boscis or suctorious tube (a, jig. 82), varying in length according to the species, is contained in this cavity, from which it can be pro truded, and the mouth is situated in tbe form of a round pore at the extremity of this proboscis. The ac tion of this tube is well dis played when a hungry Planaria makes an attack upon a Nais ; it then wraps its flat body around its prey (see fig. 76,) and applies to it the extre mity of its trumpet-shaped sucker ; the red blood of the little Anellide is seen to dis appear from the part in contact with the sucker; and if the body of the Nais be broken in the conflict, the Planaria directs the extremity of the proboscis to the torn and bleeding surface. After a meal of this kind the digestive canals of the Planaria are displayed by the red colour of their contents, like tbe corresponding parts of the Liver-fluke when filled with bile, and they greatly resemble the latter in structure ; instead of two canals, however, three are con tinued from the base of the proboscis ; one of these is central (b), and passes upwards to the anterior extremity of the body, distributing its wide cceca on either side ; the other two (c, c) descend, almost parallel to one another, and give off their ccecal processes chiefly from the outer margin, as in the Distorna. The Planarit are, equally with the parasitic Trematoda, de void of an anus : and the remains of Poly gastric infusories swallowed by them have been seen to be regurgitated by the proboscis. Mi nute nutrient vessels are continued from the extremities of the intestinal caeca, and form a very fine cutaneous network, which communi cate with a mesial and dorsal canal and two lateral vessels, as in the Diplostomum.