274 



GENERAL BIOLOGY 



a sensory fiber need not first pass to the brain before meeting the motor 

 fiber, it is called a reflex. 



The ventral nerve cord is in reality a series of ganglia, one pair lying 

 in each somite posterior to the fourth. Each pair is connected by a 

 nerve cord to the one preceding and following it. In somite four this 

 nerve cord divides into two parts, one passing on each side of the ali- 

 mentary tract to again unite 

 above the pharynx in the third 

 somite. This dorsal union is 

 the brain, while the two por- 

 tions forming it are known as 

 the circum-pharyngeal con- 

 nectives. The segmental 

 ganglia forming the nerve cord 

 are called the sub-pharyngeal 

 ganglia. The brain and ventral 

 cord form the central nervous 

 system. The nerves passing 

 from the central nervous sys- 

 tem to the various parts of the 

 body constitute the peripheral 

 nervous system. 



The supra - pharyngeal 

 ganglia supply the prostomium 



with two large nerves which give off many branches ; they also send 

 nerves into somites two and three. One nerve extends out from each 

 circum-pharyngeal connective. In each somite from four to the pos- 

 terior end of the body, three pairs of nerves arise, -two pairs from the 

 ganglionic mass and one pair from the sides of the nerve cord just be- 

 hind the septum which separates the somite from the one preceding. 



Each enlargement of the ventral nerve cord really consists of two 

 ganglia, which are closely fused together. In transverse section these 

 fused ganglia are seen to be surrounded by an outer thin layer of epi- 

 thelium, the peritoneum, and an inner muscular sheath containing blood 

 vessels and connective tissue as well as muscle fibers. Near the dorsal 

 surface are three large areas, each surrounded by a thick double sheath 

 and containing a bundle of nerve fibers. These are called neurochords- 

 or "giant fibers." Large pear-shaped nerve cells are visible near the 

 periphery in the lateral and ventral parts of the ganglion. 



The nerves of the peripheral nervous system are either efferent or 

 afferent. Efferent nerve fibers are extensions from cells in the ganglia 

 of the central nervous system. They pass out to the muscles or other 

 organs, and, since impulses sent along them give rise to movements, 

 the cells of which they are a part, are said to be motor nerve cells. The 



Fig. 169. Diagram of the Anterior End of Lum- 



bricus Herculeus to show the Arrangement 



of the Nervous System. 



I, II, III, IV. The first, second, third, and 

 fourth segments. 



1. The prostomium. 2. The cerebral ganglia. 

 3. The circumoral commissure. 4. The first ven- 

 tral ganglion. 5. The mouth. 6. The pharynx. 

 7. The dorsal and ventral pair of chaetae. "8. The 

 tactile nerves to the prostomium. 9. The anterior, 

 middle and posterior dorsal nerves. 10. The an- 

 terior, middle and posterior ventral nerves. 

 (After Hesse). 



