PEAKLY NEREIS. 161 



line running along the middle of the back. This, at 

 irregular intervals of five to fifteen seconds, contracts 

 almost to invisibility, and then expands again, taking 

 the form of a string of beads in each process. 



This, however, is only a part of the great circulating 

 system in this fine worm. In it the red blood flows 

 from the tail towards the head ; at the under side of 

 the body, there is a similar vessel (the ventral), through 

 which the blood flows from the head towards the tail 

 Besides these, there are four other longitudinal vessels 

 in the interior of the body, one above and one below the 

 intestine, and one on each side, all of which are con- 

 nected by transverse branches. Thus there is a double 

 circulation. "The great dorsal," says Dr. Williams, 

 "the reservoir of the centripetal streams of the body, 

 may be likened to a right ventricle (the lungs cut off), 

 and the great ventral to a left ventricle. The duty of 

 the former is to collect the refluent blood of the system ; 

 of the latter to circulate it again." 



Our Nereis is furnished with two pairs of dark blue 

 eyes, and with an evertile proboscis, which consists of 

 two segments. The one first protruded is beset on its 

 upper side with some largish horny points, arranged in 

 two groups, which are continued round to the under 

 side in a band of irregular lines of points. The 

 second segment has four groups of points, the groups 

 L 



