266 Memoirs of the Indian Museum. fyoi,. 1, 



Besides this notching, the dorsal setse may show also a fine fraying out of the 

 sides, such as has been already described for the Nais variabilis of this country. 

 This is often absent, sometimes hardly visible, sometimes obvious ; in one case 

 in the dorsal setae of segments ii and iii, it far surpassed that figured (for Nais 

 variabilis) in plate xvi, fig. 9, and the setae resembled nothing so much as minute 

 feathers with close-set though unconnected barbs. 



The explanation of this phenomenon I take to be the same as in the Nais 

 previously described. 



In number the dorsal setae are 2, 3, 4 or 5 per bundle; but they are not 

 all of equal length; they may be roughly classified into long, intermediate, and 

 short ; the long may be said to be those which are of the full length described above ; 

 the intermediate are a half to two-thirds the length of the long ; while the short may 

 hardly project beyond the surface of the body. 



The setal sacs are large, conspicuous, and of bulbous shape (fig. 25). Those of 

 the specially elongated setse of the third segment were very frequently noticed to be 

 quivering, or oscillating slightly and rapidly in an antero-posterior direction ; this 

 occurred when the animal was quite at rest, and when all the other setal. sacs (and 

 setae) were motionless; on seeing it in an animal whose long setae had been damaged, 

 I thought it might be due to irritation, but I soon found that it occurred also in 

 animals whose long setae were quite intact. It appears probable, therefore, that the 

 elongated setse of the third segment have a special sensory function ; and that this, 

 and not any advantage in locomotion, is the purpose subserved by their lengthening; 

 the slight vibration of the setae brought about by the continual small contractions of 

 the muscles attached to the setal sacs would be of use in exploring the surface at 

 the extremities of the setae. 



The septa are well-developed. The body-cavity contains corpuscles, which 

 contain a varying number of spherical brown bodies resembling minute drops of oil 

 {v. fig. 28) ; in each corpuscle there may be only a few — three or four — such bodies, 

 which are then of relatively large size ; or the corpuscles may appear to be made up 

 of a large number of very small brown particles. As a rule these particles are 

 larger than those of similar colour in the walls of the intestine ; they may also some- 

 times be found free in the body-cavity. White refractile particles like those in the 

 corpuscles of Nais variabilis may co-exist with the above-described brown droplets. 



Alimentary canal. — The mouth is transversely placed; the buccal cavity occupies 

 the first segment. The pharynx is protrusible ; cilia may be seen working in its 

 interior, sometimes particularly in an oval patch in its centre {v. fig. 25); its apparent 

 extent varies slightly ; it may be limited to the second segment, or may extend partly 

 or wholly through the third {v. inf., septal glands); it may have a distinctly nodular 

 appearance (figs. 25, 29). The oesophagus reaches to the end of the seventh 

 segment; it is of uniform calibre, and has the septal glands attached to it. The 

 septal glands (figs. 25, 29, 30, 38) are small masses of somewhat irregular shape, 

 situated at the sides of the oesophagus; they usually appear to rest against the 

 septum posterior to them, but sometimes no definite relation to the septa is to be 



