. Elementary text-book of zoology. Zoology. 328 The nervous system consists of two lateral longitudinal cords passing externally to the main trunks of the excretory system. They are somewhat swollen in the head, where they are connected by a trans- verse commissure; these anterior swellings and the commissure may represent a cephalic ganglion. Distinct sense organs are wanting, but the tactile sense may be ascribed to the skin, especially to that of the head and the suckers. An alimentary canal is also wanting. The nutritive fluid, already prepared for absorption, passes end
. Elementary text-book of zoology. Zoology. 328 The nervous system consists of two lateral longitudinal cords passing externally to the main trunks of the excretory system. They are somewhat swollen in the head, where they are connected by a trans- verse commissure; these anterior swellings and the commissure may represent a cephalic ganglion. Distinct sense organs are wanting, but the tactile sense may be ascribed to the skin, especially to that of the head and the suckers. An alimentary canal is also wanting. The nutritive fluid, already prepared for absorption, passes endosmotically through the body wall into the parenchyma. The excretory apparatus, on the contrary, attains a considerable development as a system of much ramified canals which are dis- tributed throughout the whole body.* It consists primarily of two longitudinal canals (a dorsal and a ventral), running along each side of the body and connected in the head and in each segment by transverse trunks. According to the state of contraction of the muscular system, these longitudinal trunks and cross branches appear sometimes straight and sometimes bent in a wavy or zigzag manner: their breadth also presents consider- able variation, so that the power of contraction has been ascribed to their walls. The longitudinal trunks only serve as the efferent ducts of a system of very fine vessels which ramify throughout the whole paren- chyma and receive numerous long- tubes : the latter begin in the parenchyma with closed funnels, which contain a vibratile ciliated lappet (fig. 264). In many cases, as in the Ligulidce and Caryo- pliyttims, these longitudinal trunks are broken up into numerous longitudinal vessels, which are connected by transverse anastomoses. In other cases, on the other hand, the two ventral vessels are enlarged at the cost of the two dorsal, which may entirely atrophy. The external opening of the excretory system is, as a rule, placed at the * Compare Tb. Pintner. '• Unter
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
