INTESTINAL R?:SPI RATION IN ANNELIDS. 823 



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 Polycha^te organisation : 

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

 Polychiete — forms, speaks of the parapodia as having become " reduced " ; so, too, Fuchs 

 (19), defining the Cirratulida), 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 pheuomonou 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 difl^erentiation 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 pai)er, 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 Oligochffite series {.-Eolosoma, Enchytrtcidse, 

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



* It has been suggested to me that the con<lition in wliich the dor.-^al vessel exists only in the anterior region is 

 probably correlated with the massing of the resi)iratory apparatus (the development of branchiiu) at the anterior end 

 of the body. The examples of the yEolosomatidiU and Enchytraeidie, however, show that the differentiation, in the 

 anterior region only, of a dorsal vessel may occur apart from such respiratory specialisation", and the same ra«y be 

 said of the Opheliidie {PotijophtluUmus) among the Polycha-ta. 



That the two conditions— pres^encc of a ditfereiiliat.-d dorsal ves.sel only in the anterior region, and development of 

 branchia: at the head end— coexist in general in the Pnlych:eta, is of course a fact, and it is highly proliable that there 

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

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

 parts of the body where the vascular system has reached a considerable degree of dillerentiation, 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 diH'erentiated throughout the length of 

 the body are not restricted to the anterior end as a site for their gills ; so, for example, ArenicoUi, the Spionida', and 

 other Polychiela, and so Bmiichiodrihis {Chxtobmnckiis) and Deru among the Naidid;e, and Branchiura among the 

 Tubificidic. 



TRANS. ROY. SOC KDIN., VOL. XLI.X. I'AH'i' 111. (NO. \A). 112 



