NEEVOUS SYSTEM OF VEEMES. 



145 



regarded as nerves. In some parts they exist as distinctly muscular 

 fibrillae. If the pouch is not developed, these fibres are attached to 

 processes of the muscular fibres, which often become converted into 

 tubes flattened at the sides. Both these conditions are found not 

 only in the same genera, but even in the same individual, where they 

 gradually pass into one another. In the last-mentioned form of the 

 muscle-cells, a large number of fibres are generally placed closely side 

 by side in the muscle-tube. The muscular fibres of the Chfetognathi 

 are distinctly striated transversely ; there are traces of such striatiou 

 in many other Vermes. 



Nervous System. 

 § 115. 



The close relation between the nervous system and the general 

 organisation is shown by the general arrangement of this system. 

 The centres and peripheral parts are simple when the body is not 

 divided into metameres ; while, when the body is segmented, meta- 

 merism is exhibited with the greatest regularity in the central organs 

 of the nervous system. In all worms the most important central 

 organs of the nervous system are placed in the 

 anterior part of the body, and generally near the 

 commencement of the alimentary canal. A de- 

 velopment of the nervous tissue from the ecto- 

 derm has been made out in several divisions at 

 least. The central organ above the fore- 

 gut is the most primitive portion of the 

 nervous system, whatever modifications 

 it may present. When a head is separated 

 off it lies in it, and always innervates the 

 sensory organs that are developed in the 

 head; it varies in the degree of its de- 

 velopment with these organs. Nerve- 

 trunks, radiating thence to the periphery 

 of the body, appear in various degrees of 

 elaboration, in proportion to the extent 

 of the area of their distribution. Two 

 different conditions may be developed from this 

 arrangement. The first consists in the ventral 

 connection of the superior central organs. This 

 gives rise to an oesophageal nerve-ring. The 

 second is distinguished by the development of two 

 longitudinal trunks, which approach one another 

 on the ventral surface, and have central elements placed in them. 



The primitive form of the nervous system is retained in most 

 of the Platyhelminthes, since they possess two large ganglionic 

 masses, connected by a transverse commissure in the anterior region 

 of the body. These " cerebral ganglia " (Fig. 62, g), with the two 



L 



Anterior 



portion of the body 

 of Mesostomum 

 Ehrenbergii. 

 g Cerebral ganglia. 

 11 Lateral nerves. 

 n ' Nerves to the ante- 

 rior end of the body. 

 dEnteron. o Mouth sur- 

 rounded by a sucker 

 (after L. Graff). 



