212 CLASS VII. 



transparency), the blood moves in the dorsal vessel from behind for- 

 wards, in the abdominal vessel from before backwards l . In the Earth- 

 worm (Lumbricus) 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 ringed-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 

 MILNE EDWARDS must not be forgotten; in Terebella 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 3 . 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 Nals, 

 where an arched vessel at the anterior extremity of the body unites the two longitudinal 

 vessels. GRUITHUISEN, Anat. der gezungelten Nalde, Nov. Act. Acad. Cces. Leap. 

 Tom. xi. p. 233. And Ueber die Nais diapkana, ibid. Tom. xiv. pp. 407, c. 



2 In Nereis : see H. RATHKE, de Bopyro et Nereide commentationes duce, 1837, 4to. 

 who calls these parts organa reticulata. MILNE EDWARDS, Ann. des Sc. nat. 2e Se"rie, 

 Tom. x. ZooL 1838, PI. 12, fig. i. Similar vascular plexuses exist also in Pleione 

 carunculata, see G. R. TREVIRANUS, Beobachtungen aus der Zool. u. Physiol. Bremen, 

 1839, s - 54* an d A. E. 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 BURDACH'S Physiologic iv. 1832, s. 143 149, may be also 



