760 PROFESSOR J. STEPHENSON ON 



the gut-wall of which it forms part ; but anteriorly the continuation of the dorsal 

 vessel, and also the lateral loops, manifest contractility, though not in anatomical 

 association with the alimentary tube ; and occasionally in Nais the dorsal vessel assumes 

 an independent contractility even in the region of the intestine. 



Branchiodrilus horteiisis (Stephenson). 



In this species (48, 51a) the dorsal and ventral vessels have much the same char- 

 acters as in Nais. Besides branching vessels to the body-wall, there are present in 

 each segment a pair of lateral loops, which in the anterior part of the body extend into 

 the gills, the limbs of the loop forming the afferent and efferent vessels of these organs. 

 The afferent vessel, springing from the dorsal vessel, is in this part of the body con- 

 tractile ; further back, where the branchial processes are small, the lateral loops do not 

 extend into them, and no part of the loops is contractile ; the lateral loops exist, how- 

 ever, though much reduced in size, as far back as the hinder end of the animal. In 

 the first five segments the lateral vessels do not present the appearance of regular 

 loops, but form an extremely irregular and complicated plexus. The blood is 

 yellowish-red. 



Relation of Contractions of Dorsal Vessel to those of the Alimentary Canal. — The 

 contractions of the dorsal vessel are in this form independent of those of the intestine 

 to a much greater degree than in any of the forms previously considered. 



The dorsal vessel may contract regularly without any accompanying diminution of 

 the lumen of the alimentary canal ; conversely, intestinal antiperistalsis may proceed 

 without occluding the lumen of the dorsal vessel. 



The two series are, as a rule, independent in rhythm ; thus the dorsal vessel may 

 be contracting thirty-two times, and the intestinal walls only seven times per minute ; 

 or the dorsal vessel may contract thirty-six, and the intestine only eighteen times per 

 minute. And here, though the rate of one was just double that of the other, the two 

 never coincided ; the character of the contractions also was different in the two series, 

 the wave of contraction of the dorsal vessel passing along much more swiftly than that 

 of the alimentary canal ; the antiperistaltic wave recurred at regular intervals, the 

 contractions of the dorsal vessel rather irregularly. 



The two series may also be independent as regards the violence of their contractions. 

 The dorsal vessel may contract actively while antiperistalsis is going on but feebly. 

 Antiperistalsis may be quite absent in the anterior part of a specimen, while the 

 vascular contractions in the same region are proceeding vigorously. An indication of 

 some slight connection between the two series was however afforded by a specimen in 

 which the vascular contraction often seemed to precede by about half a second the 

 antiperistaltic wave of the intestine. 



Anatomically, the vascular system here differs from that of the other Naididse in 

 the presence of lateral loops throughout the body, and in their extension, in the 



