. A manual of zoology. Zoology. ///. ANNELIDA: CH^TOPODA. 307 pouches of the right side from those of the left (figs. 275 and 272). The alimentary canal also shows distinctions; for while it differs greatly in the various species, it has constantly a terminal anus, while the mouth is ventral and is overhung by the preoral segment, the prostomium. Xervous system, blood-vessels, and excretory organs are influ- enced by the segmentation. The nervous system is built on the ladder plan. It begins with a supraoesophageal ganglion ('brain') lying in the prostomium, from which the c8soj)hageal commiss
. A manual of zoology. Zoology. ///. ANNELIDA: CH^TOPODA. 307 pouches of the right side from those of the left (figs. 275 and 272). The alimentary canal also shows distinctions; for while it differs greatly in the various species, it has constantly a terminal anus, while the mouth is ventral and is overhung by the preoral segment, the prostomium. Xervous system, blood-vessels, and excretory organs are influ- enced by the segmentation. The nervous system is built on the ladder plan. It begins with a supraoesophageal ganglion ('brain') lying in the prostomium, from which the c8soj)hageal commissures pass around the oesophagus to form the ventral chain, which con- sists of as many pairs of ganglia, united by longitudinal commis-. FiG. 275.—Anterior end of Nais eUnguis. 7i, cerebrum, connected by commissure with ventral chain, u; d(j, contractile dorsal, vg, ventral blood-vessel; m, muscular layer of skin; d/>, vh, dorsal and ventral chsetse; d, septa; k, prostomium; o, mouth. sures, as there are somites present. These ganglia of the ventral chain ar? closely similar, since the segmentation of the body is homonymous. There is but the slightest division of labor among the somites, and hence they differ but slightly among themselves. The prostomium always bears tactile organs and frequently eyes, which in many marine forms are highly developed, with lens, vitreous body, and retina. Otocysts are rare, but occur in diverse species. Ciliated pits (olfactory) occur on the head, goblet organs (taste) on head and trunk, and, lastly, lateral organs, sensory struc- tures of unknown function, may be metamerically arranged. The blood-vessels are most frequently represented by two main trunks which frequently (as in earthworms) contain blood colored red by ha-moglobin. One trunk runs dorsal, the other ventral, to the intestine, the two being connected by vessels (figs. 272, 276) in each segment. The blood passes forward in the dorsal vessel, backwards in the ventral. It is prop
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