ON MONOPYLEPHORUS LIMOSUS. 13 



setigerous organ there lies an apparently syncytial, deeply staining 

 mass of seta-forming cells with large nncleolatecl nuclei. The folli- 

 cular portion of the organ consists of weakly staining, somewhat 

 chitinised stroma containing spherical or ellipsoidal nuclei (fig. 8, 

 n.f.), which never enclose a nucleolus. The hypodermis of the 

 setigerous region presents nothing peculiar, except that the nuclei 

 contain each a nucleolus. The cuticle has been described above. 



The imperfectly developed setigerous organs of the posterior 

 part of the body are of the same structure as in Limnodrilus gotoi, 

 and while the young setae are developing in the follicular fundus 

 the body wall retains its ordinary condition, and there is neither 

 any depression nor trace of invagination. I therefore infer that the 

 cuticular tube surrounding each seta is a secondary secretion pro- 

 duct of the hypodermal cells and I have grave doubt of the 

 correctness of the invagination theory concerning the origin of the 

 setae. 



There are three kinds of muscles attached to the setigerous 

 organ : the interfollicular muscle is feebly developed and its fibres 

 are attached to the setae at a point between the nodule and the 

 root ; the transversal parieto-vaginal muscle fibres are arranged in 

 the plane of the cross section of the body and the longitudinal 

 parieto-vaginal ones in the horizontal plane. 



The setigerous organ is covered by a thin, feebly developed 

 peritoneum, each cell of which contains a nucleolated, unusually 

 large nucleus (fig. 8, n.pt.). 



3. Aliment aey canal. 



The mouth lies ventrally at the beginning of the first segment. 

 The buccal cavity reaches to the beginning of the second segment, 

 and in its posterior portion the lumen presents in cross section the 

 shape of a flattened W. The five layers of the buccal wall are 



