THE EXOGONE^C. 231 



usually unequally, the circular ring passing through at the level of the single 

 nucleus of the column or deeper (according to the condition of contraction?). 

 The circular fibres have their own nuclei at intervals. 



The internal epithelium and cuticle of the proventriculus, both very thin, 

 present no special features of importance, except that the former is consider- 

 ably thickened both in front where the ducts of the proventricular glands 

 open, and behind in the neighbourhood of the entrance to the ventriculus. 



The proventriculus is invested in a thin but resistant membrane which is 

 doubtless derived partly or wholly from the splanchnic layer of ccelomic 

 epithelium. It contains flattened nuclei at long intervals. In sections 

 stained with iron-hfematoxvlin, it shows a 8ne irregular network of fibrils*. 



The rest of the Exogoneic examined by me resemble Exogone fustifera in 

 the structure of the pharynx and proventriculus in all essential respects. The 

 pharyngeal and proventricular glands are present in all. In all with one 

 exception the rings of transverse fibres perforate the outer ends of the radial 

 muscle-columns, dividing them either equally or unequally. The only 

 exception is an undetermined species of Gruhea in which the rings are 

 arranged regularly between the annular rows of muscle-columns. 



The ventriculus of Exogone fustifera, somewhat shorter than the proventri- 

 culus, is nearly as broad as the latter in front, but narrows behind towards 

 the junction with the intestine. The wall of the organ contains radiating 

 muscle-columns which are more widely spaced than those of the proven- 

 triculus and of simpler structure — solid and without protoplasmic core. 

 Circular rings oE fine muscular fibres run between th« rows of columns close 

 to the outer surface. The internal epithelium does not form a definite layer, 

 but its cells lie irregularly among the muscle-columns. Here and there in 

 the wall of the organ lies a unicellular gland. '1 he ducts of these, the 

 posterior proventricular (/lands, do not open into the lumen of the ventriculus 

 but run forwards and pierce the epithelium to open into the posterior part of 

 the proventriculus. 



In Sphcerosyllis hystrix the ventriculus has a definite epithelium. In 

 Gruhea kerguelensis and 6?. quadvioculata the ventriculus is so greatly reduced 

 as to be practically obsolete. The other Exogonew studied resemble Exogone 

 fustifera in the structure of this part of the alimentary system. 



* Witli regard to the relations between the circular and radial muscular fibres in the wall 

 of the proventriculus, it is to be remarked that Malaquin takes a view which does not accord 

 ■with the account above given. He regards the annular diaphragms, complete or incomplete, 

 as always separating the rows of radial fibres, and he is thus led to the conclusion that s ucb 

 an arrangement as that occurring in Exoyone fustifera (which is common also in the 

 Syllidea) means that the radial mus le-coluums of each row are arranged" back to back." 

 But since it is exclusively with the Syllidea that he is dealing, it will be hest to defer for the 

 present entering further into this subject. 



