INTESTINAL RESPIRATION IN ANNELIDS. 753 



the dorsal vessel then proceeded rapidly forward, — much more rapidly than the gut con- 

 traction, which originated it, had travelled along the intestine. The frequency of the two 

 waves was, of course, the same, the one following on the other in quite regular sequence. 



The difference in character of the two waves, or rather of the two portions of the 

 same wave, was striking ; the one sluggish, a general antiperistalsis of the whole gut ; 

 the other rapid, confined to the dorsal vessel. Equally striking was their interconnec- 

 tion or mutual dependence, the antiperistalsis being regularly continued forwards by 

 the postero-anterior contraction of the dorsal vessel. 



Of specimens subsequently observed at Millport, most merely repeated in essentials 

 the phenomena just described. A few variations may be recounted. 



In Liimhricilhis tuba (50) the contraction of the dorsal vessel is usually, as in other 

 forms, a simple and direct continuation of the antiperistalsis of the intestine ; there is 

 no break of continuity, and the alimentary and vascular contractions may be looked on 

 as two parts of the same phenomenon. In one specimen, however, where the dorsal 

 vessel began in segment xiii., the antiperistaltic wave sometimes stopped short at 

 segment xv., so that segment xiv. formed a break between the two, and the contraction 

 of the dorsal vessel was not then directly continuous with the antiperistaltic contractions 

 of the gut. In another case the correspondence between the arrival of the antiperi- 

 staltic wave and the beginning of the contraction of the dorsal vessel was not absolute ; 

 a vascular contraction might be intercalated between the arrivals of two successive 

 antiperistaltic waves ; or the beginning of the contracticm of the dorsal vessel might 

 slightly anticipate the arrival of the antiperistaltic contraction. The most interesting 

 specimen, however, was one in which the antiperistaltic waves and the contractions of 

 the dorsal vessel were both regular, but the rhythm of the two was not quite the same ; 

 the contractions of the dorsal vessel were a little more frequent, and thus for a short 

 time now and then the vascular would seem to be a continuation of the alimentary 

 contraction ; but a longer observation shows that they were really independent. 



In the foregoing species the want of unity between alimentary and vascular con- 

 tractions is only occasional ; in Enchytrxus alhidus it seems to be more fundamental. 

 In two examples of this species the contraction of the dorsal vessel was independent of 

 the arrival of the antiperistaltic wave ; the dorsal vessel pulsated several times for each 

 antiperistaltic wave which reached the anterior end of the intestine. In another 

 specimen the dorsal vessel originated in segment xvi. ; the antiperistalsis stopped 

 abruptly at xvii., and there was an interval of a segment which showed no pulsation of 

 any kind ; the contractions of the dorsal vessel were often quite independent of the 

 arrival of the wave behind it ; the rate of the antiperistaltic contractions was only five 

 in two minutes, and the rhythm was irregular, while the dorsal vessel, on the contrary, 

 was contracting regularly four times per minute. The absence of any kind of con 

 traction in the space of a segment between the point where antiperistalsis ended and 

 where the dorsal vessel began was noticed in another case also. 



Summing up the condition in the Enchytrgeidae, we may say that both anatomically 



