NERVOUS SYSTEM. 187 



of a single filament, but it unravels its compound structure 

 chiefly in two places of its course where it meets with the 

 vulva of the female, and separates its columns to embrace 

 that orifice (Fig. 82. A. #,) and where it separates at. the an- 

 terior extremity of the trunk to encompass the resophagus 

 (Fig. 82. A. a.) These nervous columns are more obvious on 

 the large strongyli and echinorhynchi, where they present a 

 similar form and distribution. Their anterior extremities 

 generally ascend on each side of the resophagus, so as to em- 

 brace that canal more or less completely with a nervous 

 collar. The surface of the body, along which the nervous 

 columns run in these animals, and in all the other articu- 

 lated classes, is termed the ventral surface, from its being 

 the inferior in the ordinary position of the body, and from 

 the anus and other excretory orifices opening on that aspect 

 of the trunk ; the anus, the valva, and the penis are placed 

 on this surface in the entozoa. This simple form of the ner- 

 vous system presented by the lowest articulated class, re- 

 sembles the embryo form of this system in the higher classes 

 of this division, and corresponds remarkably with the first 

 perceptible form of the spino-cerebral axis in ah 1 the verte- 

 brated animals, where it appears a double white streak on the 

 outer layer of the germinal portion of the cicatricula. These 

 simple worms, consisting solely of the trunk, present also the 

 embryo-form of the whole body of the highest articulated 

 classes. In the higher animals of this class, the epizoa, 

 which adhere to some part of the external surface of aquatic 

 animals, as in the achtheres and many others, the nervous 

 system is more developed, and these animals generally 

 possess rudimentary antennae, and even eyes ; they have the 

 two longitudinal nervous chords running separately and at 

 some distance from each other, separated by the whole 

 breadth of the alimentary canal. From the want of organs 

 of sense on the anterior extremity of the body in most of 

 the entozoa, the nervous system is very imperfectly deve- 

 loped in that direction, and although it often forms a ring 

 around the ossophagus, it seldom forms a perceptible supra- 

 cesophageal ganglion where these filaments meet. 



In the rotiferous animals, where there is a complex mus- 

 cular apparatus at the anterior extremity of the trunk, for 

 the motion of the numerous large cilia, and another muscular 

 apparatus for the movements of the strong lateral maxillae, 



