NERVOUS SYSTEM OF VERMES. 151 



§ 120. 



The cerebral ganglia give off, chiefly, the nerves of the higher 

 senses, and are developed in different degrees, according to the 

 perfection of these latter. The tentacular nerves, and those of the 

 organs of sight, deserve to "be particularly noticed (Fig. 64, o). 



The nerves which rise from the ventral chain go off, as a rule, 

 from the ganglionic swellings ; in many divisions, indeed, there is 

 an apparent origin from the longitudinal commissures, but the 

 nerve may then be always referred to the nearest anterior ganglion. 

 This happens in the Scoleina, in the Siphonostoma3, in Aphro- 

 dite, as well as in the Nereidas, and others. Very often the 

 lateral branches of the ventral medulla form small ganglia, which 

 are generally placed at the base of the parapodia, and from 

 which finer nerve-twigs take their origin (e.g. in the Nereidse). 

 These ganglia are often connected together by longitudinal com- 

 missures, and thus there arises a separate part of the nervous system 

 which is co-ordinated with the ventral nerve-chord (Pleione). 



The visceral nerves are differentiated in the same way. In 

 the lower divisions of the Vermes nerves pass from the superior 

 single ganglion to the alimentary canal. This has been observed 

 in the Turbellaria as well as in the Trematoda. In the Annelides 

 not only are these nerves more developed, but they become in- 

 dependent to a certain degree, owing to the deposition of ganglia 

 in them. We divide the apparatus, which has in this way become 

 a special system of visceral nerves, into an anterior and a posterior 

 portion. The former is distributed over the oral region, and is 

 specially developed in the Chastopoda which are provided with a 

 protractile proboscis (Phyllodoce, Glycera, etc.). The posterior 

 portion, which is less developed, passes on to the enteric tube. 

 In the Hirudinea there is an azygos enteric nerve ; in the Lumbri- 

 cidas a nerve is continued from the oesophageal commissure, on either 

 side, to the ganglia placed on the enteron ; these ganglia have been 

 observed to vary in number. These two portions of the visceral 

 nervous system, notwithstanding their distribution in parts which 

 are physiologically connected, must be kept apart, for the anterior 

 portion is distributed in parts, which are movable at will, while the 

 latter alone correspond to a true enteric nervous system, and in 

 view of their physiological relation can be called a sympathetic 

 nervous system. 



Leydig, Ueber d. Nervensystem der Anneliden, Arch. f. Auat. Ph. 18G2. 

 Hermann, E., Des Centralnervensystem von Hirudo raedicinalis. Munich, 1875. 



§ 121. 



The nervous system in the Solenogastres differs in several 

 points from the forms which have been already mentioned as ob- 

 taining in the Vermes. The cerebral ganglion, which in Ohastoderma 



