INTESTINAL RESPIRATION IN ANNELIDS. 761 



anterior part of the body, into the gill-processes. The system may be said to have 

 reached a higher stage of development, and a greater degree of independence of the 

 alimentary canal, than in the preceding forms. 



Physiologically, the advance shown by the present form over those immediately 

 preceding, though it belongs like them to the Naididse, is greater than that between 

 any other two successive forms in the series here presented. Whereas hitherto the 

 main feature has been the interconnection of the two series of contractions, here, on the 

 other hand, their independence is strongly marked. In the preceding forms the gut 

 contractions appeared to be the primary phenomenon, of which the vascular contractions 

 were frequently merely a part ; here, however, the vascular contractions are the more 

 numerous, and begin to take the lead. 



TUBIFICID^. 



Clitello arenarius (Miill.). 



The circulatory system of this species has not hitherto been investigated in detail ; 

 it is probably, however, not very different in essentials from that of such a Tubificid 

 as Limnodrilus {vide post). According to my observations the ventral vessel is con- 

 tinuous throughout the body ; the supraintestinal vessel extends from segment vi. 

 anteriorly to xii. posteriorly. The hearts are two pairs, in segments viii. and ix., and 

 arise from the supraintestinal above, joining the ventral vessel below. The lateral 

 commissures, segmen tally arranged, exhibit in the posterior part of the body 

 complicated windings, and penetrate the muscular coats of the body-wall so as to lie, in 

 part of their extent, underneath the epithelium ; but they are single continuous 

 vessels and do not anastomose. In front of the middle region of the body they 

 become progressively smaller and ultimately indistinguishable (at segment xxii. in 

 an animal of 94 segments) ; in the anterior five segments of the body they again form 

 a series of complicated loops. 



Relation of Contractions of Dorscd Vessel to those of the Alimentary Canal. — Anti- 

 peristalsis, as previously stated, is always to be observed in this species, and its relation 

 to the contraction of the dorsal vessel is interesting and suggestive. The latter may 

 be illustrated by the recital of a number of observations. 



Thus in one case antiperistaltic contractions began in the segment in front of the 

 anus ; passing forwards, each became, in the sixth segment above the anus, a contrac- 

 tion of the dorsal vessel only. In another case also, each contraction was a typical 

 antiperistalsis of the gut at the posterior end of the animal ; the waves gradually 

 changed their character while passing forwards, and at the thirteenth segment from the 

 hinder end each became merely a contraction of the dorsal vessel ; no antiperistalsis 

 was seen higher up the alimentary canal, either along with or independent of the 

 vascular contractions. In still other cases the same kind of phenomenon was observed, 

 the contraction being an antiperistalsis of the gut throughout the hinder segments of 



