186 EDWIN S. GOODEIOH. 



ventral line, entering the ventral vessel of that side above the 

 nerve-cord. Dr. Eisig has accurately described and figured 

 the vascular system of Hesione.^ I can refer the reader to his 

 paper for further details. It is at the point where the dorso- 

 lateral vessel (A of Eisig) first touches the hinder edge of the 

 oblique muscles that the '' ciliated organ" is situated. 



Fig. 1 is a careful drawing of an inner view of the left side 

 of a portion of a worm hardened and cut in half. It shows 

 the position and relative size of the ciliated organ, c. o., in two 

 segments about the mid-region of the body in front of the 

 posterior extremity of the long pharynx, which has been 

 removed, together with the longitudinal dorsal vessels. 

 Below are seen the longitudinal ventral vessels {v. v. of 

 Eisig), into which enter the ventro-lateral vessels, vl. v. 

 {v. vv. of Eisig). The ciliated organ, of considerable size, is 

 crescent-shaped, with two free horns or limbs, the internal 

 one alone being visible in this drawing. The external horn 

 curls round to the outer side of the bundle of muscles. 



A microscopic examination shows that that surface of the 

 organ which faces backwards and away from the muscle is 

 furrowed with deep grooves, alternating with sharp-edged 

 ridges, running transversely to its long axis (figs. 3, 14, and 

 20). The ridges (fig. 3, c. r.) extend up to the very edge along 

 the dorsal free margin ; ventrally, on the contrary, they stop 

 short of the edge, leaving a narrow smooth strip beyond. The 

 whole of this grooved surface is densely covered with fine cilia. 



A comparison of three transverse sections (figs. 9, 10, and 11), 

 taken from before backwards, of three sagittal sections (figs. 

 14, 15, and 16), taken from within outwards, of three frontal 

 sections (figs. 20, 21, and 22), taken from above downwards, 

 and of the reconstruction in fig. 2 will make clear the structure 

 of the ciliated organ. It is free along its dorsal margin ; but 

 for about the middle third of its length the lower edge is 

 attached to the muscles by a thin septum, formed by a double 

 layer of peritoneum (figs. 3, 21, and 16, at v. e.). About half- 



^ H. Eisig, "Ueber das Vorkommen eiaes schwimmblasen ahnlichen Organs 

 bei Anneliden,'" MittheiL Zool. St. Neapel,' vol. ii, 1881. 



