304 



VEMS. 



Ct 



mented from the segmented forms, under the respective heads of 

 Vermes 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 

 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 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). 

 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. 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- 

 named layer or matrix and in the lower worms is extremely thin and 

 delicate. In the Nemathelminthes it is often laminated, and can 



FIG. 215. -Head and anterior segments of 

 Eunice seen from the dorsal surface. T, 

 Tentacles or antennae of the pra^stomium ; 

 Ct, tentacular cirri; C, cirri of the 

 parapodia ; Sr, branchial appendages 

 of the parapodia. 



