Elements of Comparative Anatomy (1878) Elements of Comparative Anatomy elementsofcompar78gege Year: 1878 EXCEETOEY OEGANS OF VEEMES. 173 be continued from them requires to be more closely examined. The relations of the canals, which are known as the water vascular system, are more exactly known in most Platyhelminthes. They have not been observed in the land Planarians. In the Trematoda, and many Turbellaria, two excretory canals, one on each side, ramify in the body; the chief trunks giving off fine branches which traverse its parenchyma (Fig. 80, A B). Long cilia are Fig. 80. Diagram of
Elements of Comparative Anatomy (1878) Elements of Comparative Anatomy elementsofcompar78gege Year: 1878 EXCEETOEY OEGANS OF VEEMES. 173 be continued from them requires to be more closely examined. The relations of the canals, which are known as the water vascular system, are more exactly known in most Platyhelminthes. They have not been observed in the land Planarians. In the Trematoda, and many Turbellaria, two excretory canals, one on each side, ramify in the body; the chief trunks giving off fine branches which traverse its parenchyma (Fig. 80, A B). Long cilia are Fig. 80. Diagram of the excretory organs of Platyhelminthes, showing the different forms, which may be derived from one another. distributed on the walls of the fine canals. The principal trunks, which are as a rule somewhat enlarged, still open in many forms in the anterior region of the body (Fig. 80, A) (Ti'istoma papillosum). The orifice (porus excretorius) is most commonly placed towards the posterior region (D), where the two vascular trunks approximate and unite at a common orifice. In this way a terminal tract, common to both canals, is formed. This tract, which is generally enlarged, has the form of a contractile vesicle {E). Vesicles of this kind may be formed even on trunks which open separately. They form a third division of the apparatus. In the Cestoda, the fusion of the excretory canals into a single porus excretorius, placed at the end of the scolex-body, which in the Other Platyhelminthes is only an acquired character, has apparently become typical. A contractile vesicle generally foi-ms the point of union. There are generally a larger number of principal trunks—that is four, six, or eight—which either unite with one another by loops in the head, or merely curve round it, and passing backwards branch again; in this case their more special characters are similar to what are found in other Platyhelminthes. When the scolex-form is divided into metameres, the terminal portion of t
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