168 THE ANNELIDE3. § 157. 



Bel. by transverse anastomoses. The peripheric vessels arise by means of a 

 capillary net-work, from the most various points of the longitudinal and 

 transverse vessels. The circulation has on the whole a determinate direc- 

 tion, — the dorsal vessels force by a kind of peristaltic movement the 

 blood from behind forwards into the ventral vessel, which returns it 

 into the dorsal vessel. The blood can, however, pass from the dorsal 

 to the ventral vessel by a much shorter way, — by traversing the 

 capillaries, or directly through the transverse anastomoses. It is, more- 

 over, very probable that the course of the blood in the transverse vessels 

 is not always in the same direction, and that it may under certain circum- 

 stances pass from the ventral into the dorsal vessel. This makes it difficult 

 to decide which of these vessels are arteries, and which veins. The respir- 

 atory organs, which usually form the limit between the venous and arterial 

 systems, are, v/ith most Annelides connected with the transverse vessels, 

 and therefore throw no light upon this doubtful point. From the nmltitude 

 of these transverse anastomoses, it must appear impossible to distinguish 

 the arterial from the venous blood, and the distinction of veins from arte- 

 ries with most Annelides must be wholly arbitrary. 



The blood of the Annelides, although red like that of the vertebrates, 

 is, however, quite different. It is composed of a liquid containing glob- 

 ules. These last, which are always colorless, of unequal size, and of a 

 spherical form, are granulated on their surface.'^' The blood liquid is 

 either colorless, or contains a coloring matter, which is usually I'ed, but 

 sometimes yellow or green. 



§157. 



With the Nemertini, whose blood is red ^'^ but as yet imperfectly known- 

 the circulation appears to be due to two cardiac dilatations concealed in the 

 cephalic extremity.'^' 



The Hirudinei have, beside the two median vessels, two lateral ones also, 

 which intercommunicate by very numerous transverse vessels.''" From the 

 contractions of these vessels, the blood is driven sometimes forwards, and 



1 For the blood-globules of the Annelides, see hearts are divided into two chambers, the anterior 

 Wai^ner, Zur vergleich. Physiol, d. Blutes, lift. I. having deep-colored blood, while that of the ixisterior 

 p. 23, lift. II. p. 39. According to him, those of one is more clear. This arrangement has led this 

 Terebella (Ibid. lift. I. fig. 8) are pale red, circu- naturalist to regard as hearts the bodies described 

 lar discs. Ilere the exception is remarkable, sup- by Rathke. with Borlaxia striata, as cerebral gan- 

 posing there was not an error of observation. It glia, and as blood-vessels, the nerves which are given 

 appears that beside the blood which circulates in oif from them laterally (see above, § 148, note 2>. 

 the vessels the fluid contained in the visceral cav- But if Quatrefa^es'' figures of the nervous and 

 ity of the Chaetopodes plays also an important vascular systems of iVe7ner?es »jrt«dii//a are e.\am- 

 part in the act of nutrition, for the eggs and the ined (Regne anim. de Cuvier t-d. iUustr. Zoophytes. 

 spermatic particles which with these animals are PI. X.'C.KIV. fig. 1), it will be seen that there are 

 often detached from the ovaries and testicles at here three main trunks, a median and two lateral, 

 a time when still quite imperfect, attain their com- These last accomiiany the lateral nerves, while it 

 plete development while remaining in the visceral bifurcating vessel which passes from the median to 

 cavity, i>robably by means of this nutritive fluid. the two lateral trunks, embraces closely, in a loop- 

 See Qaatrefai^es, Ann. d. Sc. Nat. V. 1846, p. 379. like manner, the two cerebral ganglia, ao that they 



1 According to Milne Edwards (Ann. d. So. Nat. easily escape observation. This is perhaps true 

 X. 1838, p. 197), the blood of the Nemertini is color- also of Borlasia. 



less. 3 The sanguineous system of Sartf^uisuga has 



2 The vascular system of Polystemma has been been very carefully descril>ed by Brandt (Med. 

 distinctly seen by Duiies (Ann. d. Sc. Nat. XXI. Zuol. II. p. 247, Taf. XXIX. B.) ; see also Boja- 

 1830, p. 75, PI. II. fig. 6), and by Orsted (Bes- nus, in the Isis, 1818, p. 2089, Taf. XXVI. fig. 3. 4. 

 chreib. d. Plattwiirmer, p. 17). It is composed of With Nephelis, there are only two lateral vessels 

 many longitudinal vessels, which intercommunicate and an abdominal one, lying alcjng the ventral me- 

 not by transverse ones, but by arcuate anastomo- dulla ; see Muller, in MeckeVs Arch. 1S28, p. 2A. 

 ses at the cephalic extremity, and by two hearts in Taf. I. tig. 1. 



the cervical region. According to Orsted, these 



