256 



ANNELIDA 



blood-vessels, projecting, in the adult worm Fig. 125. 



(fig. 125, 6), at the root of the setiferous pro- 

 cesses upwards and outwards one-fourth of an 

 inch from the surface of the body. They are 

 limited in number and distribution to the 

 fourteen or sixteen middle annuli or segments. 

 These external branchiae are commonly de- 

 scribed as forming simply arborescent tufts ; 

 the division of the vessels is, however, found, 

 on more minute examination, to be regulated 

 in accordance with a fixed principle. When 

 fully injected with blood, the vessels of each 

 branchia form a single flattened plane, which 

 rises obliquely above and across the body im- 

 mediately behind each brush of setse. In the 

 adult animal each gill is composed of from 

 twelve to sixteen primary branches arising 

 from a single trunk that proceeds from the 

 great dorsal vessel': the vessels in the bran- 

 chial tuft describe zigzag outlines, the se- 

 condary branches projecting from the salient 

 point or the outside of each angle of the zig- 

 zags, and the tertiary from similar points on the 

 secondary branches. This mode of division, 

 occurring in one place and in all the smaller 

 branches, results in a plexus of vessels of 

 great beauty of pattern or design. Each 

 branchial tuft and each individual vessel pos- 

 sesses an independent power of contraction : 

 in the contracted state the tuft almost dis- 

 appears, so completely effected is the empty- 

 ing of the vessels. The contraction or systole 

 in any given tuft occurs at frequent but irre- 

 gular intervals : this movement does not take 

 place simultaneously in all the branchia3, but 

 at different periods in different tufts. The 

 vessels have the appearance of being quite 

 naked; and if examined in the living state, 

 each ramuscule seems to consist of only a 

 single trunklet : if this were really the case, it 

 would of course resolve itself into a tube end- 

 ing in a cul-de-sac, and the blood-movement 

 would be a flux and reflux ; but by injection 

 it is easy to show that the finest division of 

 the branchial arbuscule contains a double Amiicoia piscator 



