INTESTINAL RESPIRATION IN ANNELIDS. 828 



group in text-book and laboratory is Nereis, a typically errant form. The parapodia, 

 the organs of active locomotion, are accepted as of the essence of Polychsete organisation : 

 thus Goodrich (22), deriving the Archiannelids from Chsetopodous — presumably 

 Polych^te — forms, speaks of the parapodia as having become " reduced " ; so, too, FuCHS 

 (19), defining the Cirratulidge, speaks of the parapodia as being reduced (reduziert) to 

 papillae. And no doubt other indications of a similar view could be collected. 



It is the other side of the argument that I here wish to present. First are the facts 

 of the distribution of intestinal respiration, as evidenced by ascending ciliary action ; 

 these formed the starting-point for the present discussion, and need not be again 

 alluded to. 



Related to this phenomenon is the fact that in certain Sedentaria (pp. 806, 807) 

 a very considerable physiological importance attaches to the posterior end of the body ; 

 the connection is illustrated further on p. 819. On Sedgwick's theory, adopted in 

 the present discussion, the physiological value of the two ends of the body would 

 be at first — before any marked differentiation in favour of the head took place — 

 approximately equal. 



Next, we may briefly notice some points in connection with the vascular system. 

 A dorsal vessel is present, as a rule, throughout the whole length of the body in errant 

 forms. In sedentary forms it is usually present only in the anterior part of the body ; 

 posteriorly there exists a perienteric sinus, from which the dorsal vessel has not become 

 differentiated ; the blood in the sinus is propelled by antiperistaltic contractions of the 

 muscular coat of the intestine. 



These features are, according to the views put forward in the present paper, in great 

 part primitive. The sinus is indeed perhaps a secondary character, and is to be looked 

 on as derived from an irregular system of inter - communicating spaces by the 

 disappearance of the partitions between them {cf. p. 778). But the non-differ- 

 entiation of the dorsal vessel in the hinder portion of the body, and the propulsion 

 of the blood in the sinus by antiperistaltic contractions of the intestine, are primitive 

 features, which occur low down in the Oligochsete series [JEolosoma, Enchytrseidse, 

 pp. 749, 752), and which represent early stages in the evolution of the vascular system.* 



* It has been suggested to me that the condition in which the dorsal vessel exists only in the anterior region is 

 probably correlated with the massing of the respiratory apparatus (the development of branchise) at the anterior end 

 of the body. The examples of the ^Eolosomatidte and EnchytrseidBe, however, show that the dilfereuLiation, in the 

 anterior region only, of a dorsal vessel may occur apart from such respiratory specialisation^, and the same may be 

 said of the Opheliidae {Polyoiihthalmus) among the Polychseta. 



That the two conditions — presence of a differentiated dorsal vessel only in the anterior region, and development of 

 bi-anchiae at the head end — coexist in general in the Polychseta, is of course a fact, and it is highly probable that there 

 is some relation between the two. But the correlation appears to me to be in a sense the reverse of that implied in 

 the above suggestion. The necessity of providing a vascular supply for the gills restricts the gills, in general, to those 

 parts of the body where the vascular system has reached a considerable degree of differentiation, with definite blood- 

 vessels : in most of the Sedentaria this condition is only fulfilled at the anterior end of the body, — hence the develop- 

 ment of gills in that region only. Forms in which definite vessels have been differentiated throughout the length of 

 the body are not restricted to the anterior end as a site for their gills ; so, for example, Arenicola, the Spionids, and 

 otiier Polychseta, and so Branchiodrilus (Cheetobranchus) and Dew among the Naidida?, and Branchiura among the 

 Tubificidse. 



TKA.NS. ROY. SOC EDIN., VOL. XLIX. PART III. (NO. 14). 112 



