INTESTINAL RESPIRATION IN ANNELIDS. 759 



nine per minute ; but at intervals an antiperistaltic wave may come along, obliterating 

 the dorsal vessel in its progress, and thus combining the two series of phenomena in 

 itself. The antiperistalsis appears to be the same thing as, although in a more violent 

 form than, the contraction of the dorsal vessel. 



(2) An antiperistaltic wave, beginning at the posterior end of the body as a con- 

 striction of the lumen of the intestine, may as it passes forwards become merely a 

 contraction of the dorsal vessel ; so that anteriorly, while the dorsal vessel is quite 

 occluded by the passage of the wave, there is no, or onl}^ the very slightest, constriction 

 of the lumen of the gut. Not only so ; but we may have a contraction which, as it 

 enters the microscopic field, has the characters of an antiperistaltic wave ; the wave 

 will cease to affect the lumen of the gut, and the ventral wall of the gut will not move, 

 — in other words, the wave passes across the centre of the field as a contraction of the 

 dorsal vessel ; finally, the whole gut will again be constricted by the wave, i.e. the 

 definitely antiperistaltic character of the phenomenon will be manifested by the 

 participation of the ventral wall of the intestine in the constriction, and the wave will 

 disappear from the microscopic field as an antiperistalsis once more. That is to say, 

 that even in the same high-power field one and the same wave may be an antiperistalsis 

 of the gut, a simple contraction of the dorsal vessel, and an antiperistalsis again. 



The above series of observations, though representing variations from what may be 

 regarded as the simpler and more typical condition, merely serve to emphasise the fact 

 that the two series of occurrences are essentially manifestations of the same phenomenon. 

 Other observations, however, illustrate a commencing tendency to dissociation of the 

 two series, and show that the contractility of the dorsal vessel is becoming, in a slight 

 degree, independent of the rhythm of the contractions of the alimentary wall. 



(3) In certain cases, though antiperistaltic movements are going on, they do not 

 seem greatly to affect the lumen of the dorsal vessel. The proper contractions of the 

 dorsal vessel are less frequent than the antiperistalsis ; in one case they coincided for a 

 short time now and then with every second antiperistaltic wave, and in the intervals 

 had a rhythm entirely their own. The impression here given was that of an essentially 

 double series of events sometimes coinciding ; not, as in previous cases, of an essentially 

 single series sometimes differentiating into two. 



(4) In one case the rhythm of both was the same in the posterior part of the 

 animal, while anteriorly in the neighbourhood of the stomach the two series were 

 independent. 



In the four genera which have just been considered, the vascular system shows a 

 greater degree of anatomical differentiation than in those previously discussed. The gut- 

 plexus is a less marked feature ; the general plan of the circulation, with distinct dorsal 

 and ventral vessels, is as in Chsetogaster, but the contractile lateral commissural vessels 

 are more numerous. The blood is red. 



Physiologically, the contractility of the dorsal vessel is closely related to that of 

 TRANS. ROY. SOC. EDIN., VOL. XLIX. PART III. (NO. 14). 104 



