. Elementary text-book of zoology. Zoology. 304 VEBMES. mented from the segmented forms, under the respective heads of Verities and Annelida. The form of the body, which is soft and adapted to live in damp media, is usually elongated, flat, or cylindrical, sometimes without rings, sometimes ringed, and sometimes divided into segments (meta- meres). In every case we can distinguish a ventral and a dorsal surface. It is on the first that the animal moves or attaches itself to foreign objects. The mouth is usually placed ventrally at the end of the body which is directed forward in locomotion. Th
. Elementary text-book of zoology. Zoology. 304 VEBMES. mented from the segmented forms, under the respective heads of Verities and Annelida. The form of the body, which is soft and adapted to live in damp media, is usually elongated, flat, or cylindrical, sometimes without rings, sometimes ringed, and sometimes divided into segments (meta- meres). In every case we can distinguish a ventral and a dorsal surface. It is on the first that the animal moves or attaches itself to foreign objects. The mouth is usually placed ventrally at the end of the body which is directed forward in locomotion. The con- trast between the flat, shorter form of body and the cylindrical and elongated seems, especially in the case of the non-segmented worms (Vermes s. str.), to be of importance, so that on this ground we can establish the classes of Platyhelminthes or flat worms, and of NemathelmintJies or round worms. The segmented worms (Annelida] possess a ventral chain of ganglia in addition to the brain, and a segmentation of the organs which corresponds more or less with the external segmentation. The portions of the body which are primitively alike and are known as segments or nietameres do not by any means always re- main homonomous. In the most highly developed segmented worms, the two anterior seg- ments unite to form a division of the body which foreshadows the head of the Arthropoda, and, like the latter, is pierced by the mouth, contains the brain, and bears the sense organs (fig. 245). Ct. PIG. Head and anterior segments of Eunice seen from the dorsal surface. T, Tentacles or antenna? of the prsestomium; Ct, tentacular cirri; C, cirri of the parapodia; Hi\ branchial appendages of the parapodia. In the succeeding metameres there are also frequently variations of form which disturb the homonomy. The skin of worms presents very different degrees of consistence, and covers a strongly developed muscular system. In the skin we can distinguish a layer of cells (hypodermis) or.
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
