XI] SIPUNCULUS 173 



nephridia " of Echiuroidea, and this was one excuse for including 

 the latter among Gephyrea. Attached to the sides of the intestine 

 near the anus are two branched outgrowths termed bush-bodies. 

 These (g, fig. 69) were formerly compared to the posterior nephridia 

 of Echiuroidea ; but they have been shown to be thickenings of the 

 visceral peritoneum whence floating cells (amoebocytes) are budded 

 off into the coelomic fluid. 



The nervous system consists of a small squarish supra-oesophageal 

 ganglion or brain which is beset by a number of curious lobes called 

 fungiform processes. Above the brain there is a ciliated canal 

 leading up to the external surface of the body this is doubtless 

 sensory in character. The ventral nerve-cord (b, fig. 69) shows no 

 ganglionic thickenings of any kind, it is connected with the brain 

 by a long "nerve-collar" which runs through the whole length of 

 the introvert. In order to avoid the risk of straining these delicate 

 cords when the introvert is suddenly shot out they are flanked on 

 either side by a special muscle which takes the strain when the 

 introvert is stretched. 



The larva of Sipunculus in many ways resembles that of 

 Phoronis. As in the latter larva, an oblique postoral band of cilia 

 is the chief organ of locomotion. There is also as in the Phoronis 

 larva a praeoral lobe, the greater part of which is eventually cast 

 off. The anus is at first terminal but becomes slowly displaced 

 on to the dorsal surface by the outgrowth of a ventral protrusion, 

 so that what takes place with cataclysmal suddenness in Plwronis 

 is effected by a gradual process in. Sipunculus. The study of 

 development therefore reinforces the arguments drawn from the 

 study of adult anatomy that the two groups of Sipunculoidea and 

 Phoronidea are akin, and it would be a great advantage if the old 

 term Gephyrea were exclusively devoted to them. 



