26 COMPARATIVE ANATOMY 



muscular loop the first loop, we find that the second loop 

 lying nearest to it contains part of the wide tube and the 

 greater part of the narrow tube, whilst the third loop contains 

 part of the wide tube, part of the narrow tube, and the whole 

 of the middle tube. All the tubes, with the possible ex- 

 ception of the muscular duct, are composed of hollow 

 "drain-pipe" cells joined end to end, and the opening of the 

 nephrostome is due to the enlargement of the lumen of the 

 drain-pipe cells, accompanied by the thinning out and eventual 

 disappearance of their walls on one side. 



The drain-pipe cells of the middle tube and wide tube are 

 glandular, and filled with excretory granules which are passed 

 in to the lumen of the nephridium and so to the exterior. 

 The opening of the nephrostome is nearly always choked 

 with broken-down corpuscles of the coelomic fluid, which are 

 gradually swept by ciliary action into the nephridial tube and 

 passed out of the body. 



The nervous system consists of a pair of supra-pharyngeal 

 ganglia lying just above the anterior end of the pharynx in the 

 third somite. They give off large nerves to the prostomium 

 and smaller nerves to the second and third segments. The 

 supra-pharyngeal ganglia are connected by a pair of circum- 

 pharyngeal connectives passing on either side of the pharynx, 

 with a sub-pharyngeal ganglion pair lying in the fourth 

 segment. From the sub-pharyngeal ganglion pair a ventral 

 nerve-cord is continued straight backwards in the mid-ventral 

 line to the posterior end of the body. In each somite the 

 ventral cord swells up to form a ganglionic enlargement from 

 which three pairs of nerves are given off, two close together 

 in the hinder part of the somite and one pair more anteriorly. 

 The nerves contain both afferent and efferent fibres, and pass 

 to the musculature and integument of the body-wall. It is 

 noteworthy that the efferent fibres originate from nerve ganglion 

 cells situated in the ganglionic enlargements of the cord, whilst 

 the afferent fibres originate from ?ense cells in the epidermis, 

 and their inner ends branch and come into contact, but do 

 not unite with the plexus of nerve fibrils in the cord. 



Though the ventral nerve cord appears to be single it is 

 really double, being composed of two longitudinal ganglionated 

 cords firmly fused together. The double nature of the cord 



