Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 UEIXAET OEOANS. 75 is mainly discharged by the special secretory organs. These have the form of glands of a simple or complex structure which originate from invaginations of the outer skin or of the intestinal wall, and consist essentially of simple or branched tubes, or of racemo; e and lobulated glands. Among the various substances which by the aid of the epithelial lining of the walls of glands are removed from the blood and some- times utilised furt
Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 UEIXAET OEOANS. 75 is mainly discharged by the special secretory organs. These have the form of glands of a simple or complex structure which originate from invaginations of the outer skin or of the intestinal wall, and consist essentially of simple or branched tubes, or of racemo; e and lobulated glands. Among the various substances which by the aid of the epithelial lining of the walls of glands are removed from the blood and some- times utilised further for the performance of various functions, the nitrogenous excretory substances are especially important. The organs by which the excretion of these ultimate products of meta- bolism are effected are the kidneys. In the Protozoa they are represented by the contractile vacuoles; in the Worms they appear as the so-called water- vascular vessels, and are constituted of a system of branched canals which take their origin in delicate internal ciliated funnels, which open into the spaces in the parenchymatous tissues or i nto the body cavity. In the latter case the ciliated funnels have a wide opening. In the Platyelminthes (flat worms) the efferent ducts of the system consist of two main lateral trunks (fig. 68, Ex.\ which frequently open together at the hind end of the body by means of a medium terminal contractile vesicle (fig. 68, ep). In the segmented worms the paired kidneys are repeated in every segment, and are known as seymental organs (figs. 69 and 70). The shett-ylands of Crustacea are in all probability to be traced back to these segments I organs: as are also the paired kidney (organ of Bojanus) of mussels, and the unpaired renal sac of Snails, both of which communicate by means of an internal opening with the pericardial division of the body cavity. In the air-breathing Arthropods and some Crustacea (Orchestia) the urinary organs are tubular appendages (Malpighian ve
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