i86 AN INTRODUCTION TO ZOOLOGY 



speaking, two cords running side by side and joined by transverse 

 bands, the commissures, and this structure becomes more apparent 

 when viewed in transverse section. It arises also as two distinct 

 cords on the ectoderm of the embryo, which become approximated 

 as they sink in from the outside, and it is to be regarded as a modifica- 

 tion of a type of nervous system quite common in invertebrate 

 animals. This sort of nervous system consists typically of two 

 parallel ventrally situated lines of ganglia linked on each side by 

 longitudinally running connectives and joined by a transverse 

 commissure joining the two ganglia in each segment. From its 

 characteristic appearance this sort of nervous system is termed the 

 rope-ladder type. It is masked in the worm by the apposition of 

 the two cords and the shortening of the commissures transversely, 

 but extension of them longitudinally. Running in the groove on the 

 dorsal side of the cord are three long fibres, the giant fibres, contain- 

 ing the axons of certain large ganglion cells, and these, together with 

 the cord itself, the ventral neural and lateral neural vessels, are all 

 bound together by one fairly tough fibrous connective tissue sheath. 

 The peripheral nervous system is composed of the nerves and the 

 sensory cells situated in the epidermis. A series of paired nerves 

 come off from the central nervous system and are distributed gene- 

 rally to the viscera and body wall. The first pair arise from the front 

 end of the cerebral ganglia and pass forwards to supply the pro- 

 stomium, being apparently mainly sensory in function. A second 

 pair start from the circum-pharyngeal connectives and supply the 

 peristomium. The third pair are large and run forward from the 

 sub-pharyngeal ganglia to be distributed partly to the peristome, 

 but mainly to the second and third somites. In each of the remain- 

 ing segments there are typically three pairs of nerves ; two pairs 

 come off close together from the ganglion in the mid somitic region, 

 and the last pair, originating in the connectives at the front end of 

 each somite, are distributed mainly to the septum just behind which 

 they arise. The sensory cells are present in the epidermis, occurring 

 singly or in groups, forming the so-called sense-buds. A large number 

 of these are present, particularly at the anterior end of the animal. 

 They lie among the ordinary epidermal cells and are arranged in 

 rings, of which there may be three in each somite, the middle one 

 containing as many as sixty buds. 



The mass of the central nervous system is composed of 

 ganglion cells and their processes, and they are in the main of two 

 sorts, although a third variety, a sensory ganglion cell, may also be 

 present, i. The motor neurons are typical nerve cells, mainly 

 pear-shaped and bipolar but sometimes multipolar, whose dendrons 

 are situated within the ventral cord. They are more numerous in 



