ON THE NEPHRIDIA OF THE POLYOH^TA. 189 



discover it in Nephthys. At that time I had only been able 

 to study poorly preserved material. After the examination of 

 a large number of living and well-preserved worms, I am able 

 to say that what I had before mistaken for the nephrostome is 

 really the "ciliated organ /^ 



In fig. 7 is given a careful enlarged drawing of the inner 

 view of the right side of some segments taken from the mid- 

 region of the body of a large hardened specimen of Nephthys 

 cseca/ and cut in half. The musculature is much more de- 

 veloped and complicated than in Hesione. Thin strands of 

 muscle stretch from the dorsal surface of the intestine out- 

 wards to the body-wall, forming an incomplete longitudinal 

 oblique septum, obi. sept. Two powerful strap-like longitudinal 

 muscles lie beneath the intestine on either side of the ventral 

 blood-vessel, v. v., and give off transverse muscles at every 

 segment. This ventral band overlies the thick muscular 

 transverse septa, sept., which extend upwards to the dorsal 

 longitudinal muscles, d. I. m. Various bundles of vertical 

 oblique muscles extend outside the main longitudinal muscles. 



The dorsal vessel, d. v., gives off a dorso-lateral vessel at 

 each segment, d.l.v. (dorso-pedal of Jaquet"), which passes 

 down to the large bundle of oblique muscles, obi. m., corre- 

 sponding to those described above in Hesione, and on which 

 are situated the ciliated organs, c. o. Thence the dorso-ventral 

 blood-vessel (branch a of Jaquet) runs downwards and inwards 

 to join the ventral subintestinal vessel, vi. v. An offshoot of 

 the ventro-lateral vessel goes to the neural longitudinal vessel 

 of that side. 



The ciliated organ when dissected out resembles somewhat 

 the shell of Pecten (fig. 19). It is smaller and more rounded 

 in shape than that of Hesione. The ciliated surface of the 

 organ faces outwards and forwards; it is raised into about 

 twenty sharp ridges, alternating with deep grooves, which 



* A specimen from St. Andrew kindly given to me by Dr. W. B. 

 Benham. 



2 M. Jaquet, "Recherches sur le Systeme vasculaire des Annelides," 

 •Mittheil. Zool. St. Neapel.'vol. vi, 188G. 



