REPRODUCTION BY GEMMATION. 253 



which one goes to the feet, the other to the intestine, and the third to 

 the branchiae, from which the blood returns into the dorsal vessel, 

 which in this worm, accordingly, carries arterial blood. The sudden- 

 ness of this division favours the imprisonment of a drop of blood in the 

 first stage of the vessels, the drop thus enclosed occasioning a bulged 

 enlargement ; but this appearance is altogether accidental. The blood 

 is admitted into and returned from the branchiae by alternate movements 

 of contraction and dilatation : these movements are not simultaneous 

 in all the branchiae, but various and independent in each individually, 

 the afflux into one being synchronous with the efflux of blood from 

 those contiguous. This contractile power is by no means peculiar to 

 these vessels. The motion of the blood in the vessels of every part of 

 the body of the Annelid is effected, not through the agency of uniformly- 

 travelling undulatory contractions of their coats, but by complete con- 

 tractions and relaxations of successive portions of the tube ; so that, 

 during the instant of contraction, the cylinder of the vessel in the part 

 contracting is completely emptied of blood, the sides collapsing and 

 meeting in the axis ; and during the period of dilatation the same por- 

 tion of the vessel becomes densely distended with blood : and this is the 

 true mechanism of the circulation in those species, even, in which a 

 central propulsive organ exists for example, in Neds and Arenicola* 

 In no part of the system, therefore, is the superadded contractile bulb 

 required as an agent of circulation, since this contractile power re- 

 sides in every part of every vessel in virtue of the muscularity of its 

 parietes. 



(660.) A general survey of the circulation in Eunice will suffice to 

 satisfy the physiologist that no part of the system contains pure arterial, 

 and no part pure venous blood. Into the double dorsal trunk arterial 

 blood is poured from the branchiae ; but to the same trunk the intestinal 

 branches contribute venous blood : the mingling of these two classes of 

 currents in the same trunk must result in blood of an intermediate 

 quality. It is, then, manifest that the great subneural trunk, which in 

 this worm is both systemic and branchial, must distribute blood of com- 

 position intermediate between venous and arterial. No part of the 

 circulatory apparatus therefore contains pure arterial blood except the 

 efferent branches of the branchiae. 



(661.) In some Annelidans we still find that gemmation performs a 

 very important part in the reproductive process : the multiplication of 

 the individual segments of the body depends entirely upon this mode of 

 increase. But this is not all : it not unfrequently happens that when 

 these animals have attained their full growth, a constriction becomes 

 apparent near the posterior extremity, immediately behind which a 

 proboscis and eyes are developed, forming the head of a new animal 

 subsequently to become separated by spontaneous fissure (fig. 124) ; 

 and even as many as six of these strangely-formed offsets have been 



