168 THE ANNELIDES. *5> 15 



Kul 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 J)lood 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. Tiiis 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, with most Annelidcs connected with the transvei'se vessels, 

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

 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 red, 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 ths Annelides, see hearts are divided into two chambers, the anterior 

 Warner, Zur ver^leich. Physiol, d. Blutes, Hft. I. havin,;,' de(:p-coljred blood, while that of the posterior 

 p. 23, ilti. il. p. 39. According to him, those of oue is more clear. This arrangement has led this 

 Terebelia (Ibid. Hft. I. fig. 8) are pale red, circu- naturalist to regard as hearts the l)(>dies described 

 lar discs, tiere the exception is remarkable, sup- by Rathke. with Borlasia striata^ as cerebral gan- 

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

 appears that beside the blood which circulates in off from them laterally (see ab;.ve, J 14S, note 2). 

 the vessels the fluid contained in the visceral cav- But it' Quatre/ag'es'' figures of the nervous and 

 ity of the Chaetopodes plays also an important vascular systems of iVemeriPs jji'JhcJiYA'i are exam- 

 part in the act of nutrition, for the eggs and the ineJ (Regne a'lim. de Cuvi^'r cd. illustr. Z'Kjphytes. 

 spermatic particles which with these animals are PI. X.V\.IV. fig. 1), it will b; s»en that there are 

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

 a time wlien still quite imperfect, attai.i their com- These last accompany the laler.il uerve.-i, while a 

 plete develipment while remaining in the visceral bifurcating vissel which passes from the median to 

 cavity, probably by means of this nutritive fluid. the two latei'al trunks, embraces closely, in a loop- 

 See Quatrefai^es, Aim. d. Sc. Nat. V. 1810, p. 379. like manner, the two cerebr.d :;ri.igua, S) that t'.iey 



1 According to Mi/nc Edwards (Ann. d. Sc. 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 SaH/^ihii^n has 



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

 distinctly seen by Dui:es (Ann. d. Sa. Nat. XXI. Zool. 11. p. 247, Taf. XXIX. U.) ; see also Boja- 

 1830, p. 75, PI. II. lig. 0), and by 0.'-.s/(.-(/ (lies- «)(,<!, in the Isis, lSlS,p. liUS.t, Taf. XXVI. fig. 3. 4. 

 chreib. d. Plattwurmer, p. 17). It is comjiosed of With Nepke/is, there are only two lateral vessels 

 many longitudinal vessels, which intercomnmnicate and an abdominal one, lying along the ventral me- 

 not by transverse ones, but by arcuate anastomo- duUa ; see Muller, in McckeC/: .\rch. 1828, p. 24, 

 ges at the cephalic extremity, and by two liearts in Taf. 1. fig. 1. 



the cervical region. According to Orstcd, these 



