Elementary text-book of zoology, general part and special part: protozoa to insecta . elementarytextbo00clau Year: 1892 304 mented from the segmented forms, under the respective heads of Vermes and Annelida. The form of the body, Avhich is soft and adapted to hve 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
Elementary text-book of zoology, general part and special part: protozoa to insecta . elementarytextbo00clau Year: 1892 304 mented from the segmented forms, under the respective heads of Vermes and Annelida. The form of the body, Avhich is soft and adapted to hve 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 Xemathelminthes 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 metameres do not by any means always re- main homonomovis. 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). In the succeeding metameres there are also frequently vai-iations of form which disturb the homonomy. The skin of worms presents very different degrees of consistence, and covers a sti-ongly developed muscular system. In the skin we can distinguish a layer of cells {Jiypodermis) or, at any rate, a nucleated layer of protoplasm which functions as a matrix, and a superficial homogeneous cuticular layer which is secreted by the first
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