INTESTINAL RESPIRATION IN ANNELIDS. 751 



y^olosoma viride Stephenson. 



Anatomy of Circulatory System. — This agrees with what has been stated for 

 A. hemprichi, with the exception that the dorsal vessel has attained a somewhat higher 

 degree of differentiation. A definite dorsal channel can sometimes be distinguished 

 in the alimentary wall as far back as the anterior part of the intestine ; and the dorsal 

 vessel in the oesophageal region (" heart ") is throughout its length always a definite 

 tube, with a clear lumen uninterrupted by strands. 



Relation of Contractions of Vascular System to those of the Alimentary Canal. — 

 The relation of the contractions of the system of vascular spaces in the gut- wall to the 

 antiperistalsis of the alimentary tract is the same as that which obtains in A. hem^prichi. 

 The dorsal vessel, where it appears as a channel in the alimentary wall, is not 

 independently contractile apart from the movements which take place in the wall as 

 a whole. The ventral vessel on the intestine is neither itself contractile, nor is its 

 diameter at all affected by even well-marked antiperistaltic movements of the intestine ; 

 it would seem, therefore, though I have no direct observations on the point, that the 

 vessel must lie outside the muscular layer of the intestinal wall. 



The contractions of the " heart " furnish an interesting series of variations, in part 

 analogous to those described above for A. hemprichi. The most usual condition, here 

 as there, is that the contractions of the "heart" are postero-anterior in direction, and 

 form a continuation forwards of the antiperistaltic waves of the stomach, recurring 

 fairly regularly like these latter at intervals of a second, a second and a half, or two 

 seconds. In addition, the following variations occur : — 



(1) The contractions of the "heart," though recurring regularly, and at the same 

 rate as the antiperistaltic contractions of the stomach, may not immediately follow on 

 the arrival of the waves at the anterior end of the stomach, but may occur in the 

 intervals between the arrival of two successive waves. 



(2) The two rhythms may be partially or completely independent. As an 

 example of partial independence, it was observed that if the antiperistaltic wave reaches 

 the anterior end of the stomach shortly before a contraction of the "heart" is due, it 

 evokes a contraction, and is fchus continued forwards in the typical manner ; if, however, 

 the antiperistaltic wave reaches the anterior end of the stomach soon after the "heart" 

 has completed a contraction, no effect is produced, the stimulus being apparently 

 applied during a " refractory period." 



(3) Instead of proceeding in a wave-like manner, the contraction of the "heart" may 

 be practically simultaneous along its whole length. 



(4) The contraction may be initiated not at the posterior end of the " heart," but at 

 some distance, a third or a half of its length, from its posterior end ; the contraction 

 then spreads in both directions from this point, forwards along the anterior, backwards 

 along the posterior part of the " heart." 



(5) The contractions of the " heart" may be definitely antero-posterior in direction ; 

 TRANS. ROY. SOC. EDIN., VOL. XLIX. PART III. (NO. 14). 103 



