ANNULATA. 241 



eacli brush of seta?. In the adult animal each gill is composed of 

 from twelve to sixteen primary branches, proceeding from a single 

 trunk, which arises fi-om the great dorsal vessel ; the vessels in the 

 branchial tuft describe zigzag outlines ; the secondary bi'anches pro- 

 ject 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 plane, and in all the smaller 

 branches, results in a plexus of vessels of extreme beauty of design. 

 Each branchial tuft and each individual vessel possess an inde- 

 pendent power of contraction ; in the contracted state the tuft almost 

 entirely disappears, so completely effected is the emptying of the 

 vessels. The contraction, or systole, in any given tuft occurs at fre- 

 quent but irregular intervals ; this movement does not take place 

 simultaneously in all the branchiae, but at different periods in differ- 

 ent tufts. As there is no heart-like dilatation in the afferent vessels 

 of the branchi*, the contractile power with which the exposed 

 branches are endowed, becomes an important means of reinforcing 

 the branchial circulation. The vessels appear quite naked, and if 

 examined in the living state, each ramuscule seems to consist only of 

 a single trunklet ; if this were really the case, it would of course re- 

 solve itself into a tube ending in a cul-de-sac, and the blood move- 

 ment would be a flux and reflux ; but by injection it is easy to show 

 that the finest division of the branchial arbuscle contains a double 

 vessel, enveloped in a common muscular, although extremely diapha- 

 nous sheath. That these vascular sheaths, which are only fine pro- 

 ductions of the integuments, are furnished with voluntary muscular 

 fibres, is proved by the rapid and simultaneous retraction of all the 

 branchiaj into the interior of the body, which follows when the animal 

 is touched. 



" The genus Eunice presents another and different type of bran- 

 chial vessels. Arranged in a prominent row of bright vessels, stand- 

 ing erect as minute combs at the dorsal base of each foot in the body, 

 the branchiae impart to all the species of this genus a graceful and 

 characteristic appearance. In every species the branchial vessels 

 divide on a uniform plan peculiar to this genus. The primary trunk 

 rises vertically along the inner side of the branchia, and sends off 

 from its outer side, at intervals, straight vessels, which gradually 

 decrease in size from below upwards ; each branch forms a straight 

 undividing vessel, curving gently upwards and towards the median 

 line : these branches become in their number characteristic and dis- 

 tinctive of species. In some of the smaller species inhabiting the 

 British coasts, the branchiai are composed only of a single vessel j 



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