IV VERMES— NERVOUS SYSTEM 225 



course it gives off, just as in the Echiuridce, corresponding right arid left 

 lateral nerves, which, running in the body cavity, unite dorsally to 

 form nerve rings (at least in Sipimmlus). Each pair of nerves corre- 

 sponds with one of the circular muscle bundles and with one of the 

 more or less distinct rings of the outer integument. In the Priapulidm 

 it is said that there are weak but regularly recurring swellings of 

 the ventral chord which correspond ■ndth the circular muscle bands. 

 While in the Sipunculidce the central nervous system lies in the body 

 cavity, in the Priapulidce it is in close connection with the hypodermis. 



Whereas in the JEcMuridce the ventral chord is distinctly segmented in the early 

 stages of development, sucli a segmentation is wanting in the Squtnculacea even in 

 those stages. The ventral strand of the latter is nevertheless homologous with that 

 of the Echiuridce and with the ventral chord of the Annulata in general. The ^iSrve 

 rings of the Sipunculacea evidently correspond with those of the Echiuridce. There 

 can be no doubt as to the homology of the brain and oasophageal ring. We have 

 also seen that the ventral chord, even in true Chcetopoda, may present no ganglionic 

 swellings, and may fuse into a single median ventral strand. It is a question whether 

 the nerve rings of the Echiuridce and Sipunculacea are repeated segmentally. In 

 the Echiuridce several body rings with smaller papillfe alternate with one with 

 larger papillee. The setee only occur on the rings which carry the larger 

 papilla. We also find that the nerve rings in the body rings with the larger 

 dermal papillje, are stronger than the others. It seems probable that several rings 

 go to one segment, and that the number of these rings with larger papillae and 

 stronger nerve rings corresponds with the true number of the segments. In the 

 Ouetopoda also (apart fi'om the Rirudinea) one segment is sometimes externally 

 divided into 2 or more rings, and more than one ganglion of the ventral chord 

 may lie in one segment. In the Hermellidie and Serpulidcc we find typically 

 2 pairs of ganglia in each segment, and further 2 transverse commissures and 2 

 lateral branches on each side, which ascend in the body wall towards the dorsal 

 middle line. 



The nervous system in other Prosopijgia is remarkably weakly 

 developed. The ventral chord is reduced to the infra -oesophageal 

 ganglion, and even this may be wanting. The small development of 

 the nervous system is probably to be traced to the stationary manner 

 of life, which results in a reduction of the specific sensory organs, 

 and — by the development of tube-dwellings, shells, or cases — of the 

 general body musculature (dermo-muscular tube). 



Phoronis possesses a nerve ring surrounding the mouth at the base 

 of the tentacles. The anus lies outside the nerve ring. From the 

 dorsal part of the ring a nerve arises which runs bac^cwards asym- 

 metrically on the left side, through about two-thirds of the length of 

 the body, and with a tube (notochord 1) passing through it. The 

 morphological significance of this nerve is unknown. The whole 

 nervous system lies in the hypodermis outside the basal membrane. 



The nervous system of the Bradiiopoda (Fig. 150) is weakly 



developed. It consists of a delicate oesophageal ring, whose upper 



portion is only very slightly swollen into a supra-oesophageal ganglion. 



The infra-oesophageal ganglion is indicated by a thickening of the 



VOL. I Q 



