ZY CLASS VII. 
transparency), the blood moves in the dorsal vessel from behind for- 
wards, in the abdominal vessel from before backwards. In the Earth- 
worm (Lwmbricus) the two trunks are united in the anterior part of 
the body by five or more (7—9) arches widened like strings of 
pearls. (It is almost impossible not to recall here the vascular arches 
which in the embryos of mammals run along the branchial fissures.) 
In others the connexion forwards is effected by vascular plexuses 
(retia mirabilia)*. The dorsal vessel is usually considered to be 
arterial, the abdominal venous: and in most of the rmged-worms 
this opinion is not without ground, as might indeed have been con- 
cluded from analogy with other articulates. Sometimes the anterior 
part of the dorsal vessel becomes wider, resembling a rudiment of a 
heart, which then is in most cases an arterial heart like that of 
spiders and crustaceans. The exception however observed by 
Mitne Epwarps must not be forgotten; in Zerebella the heart 
drives the blood to the gills, and must therefore be considered to be 
a venous heart, analogous to that of fishes. 
Other less important modifications of the vascular system consist 
in the breaking up of the two main stems into several, which are 
sometimes quite separate from each other, though placed in proxi- 
mity (Nephthys, Eunice), or in the presence of lateral longitudinal 
stems. In Pleione carunculata there are as many as seven longitu- 
dinal stems: four on the ventral surface, of which the middle ones — 
are small and lie at the sides of the nervous system, and the two 
outer which are larger and give twigs to the gills, and three on the 
dorsal surface, of which the two lateral receive the blood from the 
gills, and are connected by transverse branches with the third or 
median trunk’. In the Leech there are four principal stems, one 
dorsal, one abdominal, and two, larger than these, lateral. 
1 In this simple fundamental form the vascular system presents itself in Nais, 
where an arched vessel at the anterior extremity of the body unites the two longitudinal 
vessels. GRUITHUISEN, Anat. der geziingelten Naide. Nov. Act. Acad. Ces. Leop. 
Tom, XI. p. 233. And Ueber die Nais diaphana, ibid. Tom. xiv. pp. 407, &e. 
2 In Nereis: see H. RatuKe, de Bopyro et Nereide commentationes due, 1837, 4to. 
who calls these parts organa reticulata. Mi~neE Epwarps, Ann. des Se. nat. 2e Série, 
Tom. x. Zool. 1838, Pl. 12, fig. 1. Similar vascular plexuses exist also in Pleione 
carunculata, see G. R. TREVIRANUS, Beobachtungen aus der Zool. u. Physiol. Bremen, 
1839, s. 54, and A. KE. Grube, De Pleione carunculata Diss. Zootom. Regiomonti 
Prussor. 1837, p. 19. 
3 GruBE, De Pleione carunculata, pp. 18, 19. On the circulation in the ringed- 
worms I. MUELLER in Burpacn’s Physiologie tv. 1832, s. 143—149, may be also 
