168 THE ANNELIDES. $ 157, 
sel 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, with most Annelides «viinected with the transverse 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. 
ules. 
It is composed of a liquid containing glob- 
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 the Annelides, see 
Wagner, Zur vergleich. Physiol. d. Blutes, Hft. I. 
p. 23, Hft. II. p. 39. According to him, those of 
Terebella (Ihid. Hft. I. fig. 8) are pale red, circu- 
lar discs. Here the exception is remarkable, sup- 
posing there was not an error of observation. It 
<ippears that beside the blood which circulates in 
the vessels the fluid contained in the visceral cav- 
ity of the Chaetopodes plays also an important 
part in the act of nutrition, for the eggs and the 
spermatic particles which with these animals are 
often detached from the ovaries and testicles at 
a time when still quite imperfect, attain their com- 
plete development while remaining in the visceral 
cavity, probably by means of this nutritive fluid. 
See Quatrefages, Ann. d. Sc. Nat. V. 1846, p. 379. 
1 According to Milne Edwards (Ann. d. Sc. Nat. 
X. 1838, p. 197), the blood of the Nemertini is color- 
jess. 
2 The vascular system of Polystemma has been 
distinctly seen by Duges (Ann. d. Sc. Nat. XXI. 
1830, p. 75, Pl. IL. fig. 6), and by Orsted (Bes- 
chreib. d. Plattwiirmer, p. 17). It is composed of 
many longitudinal vessels, which intercommunicate 
not by transverse ones, but by arcuate anastomo- 
ses at the cephalic extremity, and by two hearts in 
the cervical region. According to Orsted, these 
hearts are divided into two chambers, the anterior 
having deep-colored blood, while that of the posterior 
one is more clear. This arrangement has led this 
naturalist to regard as hearts the bodies described 
by Rathké, with Borlasia striata, as cerebral gan- 
glia, and as blood-vessels, the nerves which are given 
off from them laterally (see above, § 148, note 2). 
But if Quatrefages’ figures of the nervous and 
vascular systems of Nemertes mandilla are exam- 
ined (Régne anim. de Cuvier éd. illustr. Zoophytes. 
Pl. XXXIV. fig. 1), it will be seen that there are 
here three main trunks, a median and two lateral. 
These last accompany the lateral nerves, while a 
bifurcating vessel which passes from the median to 
the two lateral trunks, embraces closely, in a loop- 
like manner, the two cerebral ganglia, so that they 
easily escape observation. This is perhaps true 
also of Borlasia. : 
3 The sanguineous system of Sanguisuga has 
been very carefully described by Brandt (Med. 
Zool. TI. p. 247, Taf. XXIX. B.)'; see also Boja- 
nus, in the Isis, 1818, p. 2089, Taf. XXVI. fig. 3.4. 
With Nephelis, there are only two lateral vessels 
and an abdominal one, lying along the ventral me- 
dulla ; see Muller, in Meckel’s Arch. 1828, p. 24, 
€af. I. fig. 1. 
