. Elementary text-book of zoology. Zoology. ANNELIDA. 365 Organs of locomotion. Special organs of locomotion may either have the form of bristle-bearing unjointed appendages (parapodia) on each ring of the body (' 'In*/i>,,<lu), or of terminal suckers (Hirudinea). In the first case each segment may possess a dorsal and ventral pair of appendages (the neuropodia and notopodia), which, however, are sometimes replaced by simple seta? embedded in dermal pits. Alimentary canal. The mouth is placed on the ventral surface at the anterior end of the body, and leads into a muscular pharynx, which
. Elementary text-book of zoology. Zoology. ANNELIDA. 365 Organs of locomotion. Special organs of locomotion may either have the form of bristle-bearing unjointed appendages (parapodia) on each ring of the body (' 'In*/i>,,<lu), or of terminal suckers (Hirudinea). In the first case each segment may possess a dorsal and ventral pair of appendages (the neuropodia and notopodia), which, however, are sometimes replaced by simple seta? embedded in dermal pits. Alimentary canal. The mouth is placed on the ventral surface at the anterior end of the body, and leads into a muscular pharynx, which is often provided with a powerful armature and can be protruded like a proboscis. This is followed by the gastric region of the gut, which occupies the greatest portion of the length of the body, and is either regularly constricted in correspondence with the segments, or possesses lateral diverticula ; it is only coiled in excep- tional cases. The amis is usually dorsal at the hinder end of the body. The nervous system consists of a cerebral or supra - cesophageal ganglion, which is derived from the apical plate of the larval prte-oral lobe, of an cesophageal ring, and of a ventral cord or ganglionic chain, the two halves of which lie moi-e or less approached to each other in the median line. The ventral cord arises from two lateral nerve cords, which probably correspond to the lateral nerve trunks of the Ne- mertines. These two cords are continuous with the cesophageal commissures, and, like the latter, are uniformly covered with ganglionic cells. This form of the nervous system may persist, as may also its ectodermal position (Archiannelida, Protodrilus) (tig. 295). In most Annelida, however, this is only a transitory condition; for at a later stage the lateral cords become separated from the ectoderm, come together in the median line, and acquire a segmentation corresponding to the metaineres of the body. The nerves of the sense organs arise from the cerebral ganglion ; the
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
