398 T)r. T. Williams on the Mechanism of Aquatic 



tirely destitute of these motive appendages. Nothing but a coiTeet 

 conception of the nature and capabilities of the chylaqueous 

 fluid M'ill enable the physiologist to unriddle this apparent 

 paradox. 



The Dorsibranchiate order comprehends a considerable propor- 

 tion of the class Annelida. " lis ont leur organes et surtout leur 

 branchies distribues a-peu-pres egalement le long de tout leur 

 corps, ou au moins de sa partie moyenne," says Cuvier. Are- 

 nicola represents the central genus. In this worm respiration 

 is performed by means of naked blood-vessels projecting at the 

 root of the setiferous processes upwards and outwards one-fourth 

 of an inch, in the adult worm, above the plane of the surface. 

 They arc limited in number and distribution to the fourteen or 

 sixteen middle annuli of the body. They are commonly de- 

 scribed as forming an arborescent tuft ; the division of the 

 vessels is, however, regulated by order and symmetry. When 

 fully injected, the vessels of each branchia form a single plane, 

 rising obliquely above and across the body, and immediately be- 

 hind each brush of setae. In the adult animal each gill is com- 

 posed of from twelve to sixteen primary branches, proceeding 

 from a single trunk which arises from the great dorsal vessel : 

 the vessels in the branchial tuft describe zigzag outlines ; the 

 secondary branches project from the salient angle of each zigzag. 

 This mode of division, occurring in one plane and in all the 

 smaller branches, results in a plexus of vessels of extreme beauty 

 of pattern, — a captivating example of symmetry amid irregularity, 

 harmony amid lawless variety. Each branchial tuft and each 

 individual vessel possess an independent power of contraction ; 

 in the contracted state the tuft almost entirely disappears, so 

 completely effected is the emptying of the vessels. The contrac- 

 tion or systole in any given tuft occurs at frequent but irregular 

 intervals ; this movement does not take place simultaneously in 

 all the branchiae, but at different periods in different tufts. As 

 there exist no heart-like dilatations in the afferent vessels of the 

 branchiee, 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 resolve itself into a tube ending in a cul-de-sac, and 

 the blood movement would be a flux and reflux ; but by injec- 

 tion it is easy to show that the finest division of the branchial 

 arbuscle contains a double vessel, enveloped in a common mus- 

 cular though extremely diaphanous sheath. That these vascular 

 sheaths, which are only fine productions of the integuments, are 

 furnished with voluntary muscular fibres, is proved by the rapid 



