798 PROFESSOR J. STEPHENSON ON 



Nereids. 



Nereis pelagica. 



I examined at Millport a number of small specimens of Nereis, which belonged to 

 the common species of the place, and which appeared to correspond most closely to 

 N. pelagica. 



Except in one instance, no reversed ciliary motion in the intestine was observed, 

 though the anus might be open. The exception was a headless specimen, possibly 

 though not certainly of the same species, in which a faint ascending ciliary action was 

 observed in the posterior part of the intestine, the anus being open. 



On the other hand, antiperistalsis, though in a few it might for a time be in abeyance, 

 was noted in every specimen examined. The contractions may be regular and strong ; 

 in one case, with a rhythm of six per minute, the waves progressed slowly forwards, 

 taking about four seconds to traverse one segment ; during the first part of this observa 

 tion the contractions started not at the anus but at the seventh segment from the 

 posterior end ; later, however, a series of extremely active contractions began quite at 

 the hinder end of the animal. In the headless specimen mentioned above, antiperistalis 

 was regular, strong, and very evident ; the waves proceeded at the rate of eleven per 

 minute, and could be followed for thirty-two or more segments from the posterior end. 

 In other cases the contractions, beginning at the anus, may be well marked and regular, 

 but may be limited to a few segments only. 



In some specimens antiperistalsis is less well marked. Though easily visible, it 

 may be gentle and discontinuous. In other cases the contractions may be slight or 

 hardly noticeable, irregular in rhythm and limited in extent, and may occur some 

 distance in front of the anus. In two specimens the movements were seen to cease 

 altogether for a time. 



The relation of the contractions of the dorsal vessel to the antiperistaltic waves is 

 variable. Asa rule none exists. The antiperistalsis may be feeble, or irregular, while 

 the dorsal vessel is beating strongly and regularly. The contractions of the dorsal 

 vessel persist when antiperistalsis is temporarily in abeyance. Or though both series 

 of contractions may be proceeding regularly and strongly, they may be quite independent 

 of each other ; those of the dorsal vessel may be three times as frequent, and much 

 more rapid in their passage. 



In one case, however, where the antiperistalsis began at the anus, and ceased as such 

 a few segments in front of this, it appeared to be continued forwards as a contraction 

 of the dorsal vessel ; the wave propagated itself forwards not along the intestinal walls, 

 but along those of the dorsal vessel, as happens regularly in j^olosoma and the 

 EnchytraiidjB [cf. Part I.). The connection between the two series of contractions, 

 intestinal and vascular, was maintained for periods of perhaps half a minute, and then 

 would be broken, to be resumed again shortly afterwards ; and so on. The rhythm of 

 the contractions was fairly rapid — about once in four seconds. 



