$ 158. THE ANNELIDES. 171 
These last are sometimes found alone. With many of these An- 
nelides, the transverse vessels are dilated, before branching, into real 
branchial hearts.“ As their branchiae are variously situated among the 
transverse anastomoses, the distinction between the arterial and venous 
blood is not as marked as with the Capitibranchiati; it must be arbitrary, 
as with the Hirudinei and Abranchiati. 
The blood is usually red, but. 
sometimes is yellow or nearly colorless. 
CHAPTER VII. 
RESPIRATORY SYSTEM. 
§ 158. 
With the various families of the Annelides, the respiratory organs are 
formed after wholly dissimilar types. 
With the Nemertini, they are least developed, for, excepting two longitu- 
dinal fossae upon the sides of the cephalic extremity,” there are no organs 
which can be regarded as of this nature. 
These two respiratory cavities are of variable depth, and their lateral 
borders are so approximated as to have the aspect of a longitudinal open- 
ing, and with some they are situated so far out on the cephalic extremfty: as 
to be blended together.” They are lined with a delicate ciliated epithe- 
lium, quite different from that covering the rest of the body,® and by the 
vortex actions of which, fresh water is brought constantly in connection 
with the blood.” 
Considering the smallness of these organs, it is very 
probable that the whole skin has also a respiratory function.. 
18 Arenicola ; see Milne Edwards, loc. cit. Pl. 
XII. 
19 Eunice ; Ibid. Pl. XII. fig. 2. 
20 With Eunice, Nephtys,Glycera, and Areni- 
cola, the blood is red; with Phyltodoce, it is yel- 
low; and it is nearly colorless with Aphrodite, 
Polynoz, and Sigalion ; see Milne Edwards, loc. 
cit. p. 196.* 5 
1 See Muller, Zool. Danica. Tab. LX VIII. fig. 
1-4 (Tetrastemma (Planaria) viride); Delle 
Chiaje, Memor. loc. cit. Tay. LXXVIUI. fig. 8, a 
tie geniculata); Quoy and Gaimard, Atlas 
ol. de Astrolabe Zooph. Pl. XXIV. fig. 10 (Bor- 
dasia viridis) ; and the Dict. d. Sc. Nat. LVI. Art. 
Vers, p. 574, Pl. Parentomozoaires, Nemerteés, fig. 
1, 2 (Borlasia Angliae, and Cerebratulus biline- 
atus); also Huschke, Isis, 1830, Taf. VIL. fig. 1-3. 
Notospermus drepanensis. 
2 Tetrastemma viride, Polia geniculata, and 
Micrura fasciolata (Ehrenberg, Symb. phys. 
Phytozoa Tab. IV. fig. 4. e. i. g.). 
3 See Quatrefrges, Régne anim. illustr. Zooph. 
Pl. XXXIV. fig. 1, b. b. (Wemertes Camillae). 
4 Rathké (see above, § 149, note 1) is of the opin- 
ion that these two cephalic fossae are the seat of 
touch ; but the view of Orsted (Beschreib. d. Platt- 
wirmer, p. 18, 77), who thinks them of a respira- 
tory nature, is, perhaps, the more correct. In sup- 
port of this last, is the fact of the presence of cili- 
ated epithelium, and of a very large blood-vessel 
directly beneath them (see Quatrefages, loc. cit. 
Pl. XXXIV. fig. 1, g. g. (Nemertes Camiliae) ) 
and which, in many Nemertini, is clearly seen 
through the thin epithelium ; see.Mudler, Zool. 
Dan. Tab. LX VILLI. (T'etrastemma vivire) ; Delle 
Chiaje, Memor. Tav. LXXVIII. fig. 8 (Polie 
geniculata), and Isis, 1830, Taf. VII. (Yotosper- 
mus drepanensis). 
* [ § 157, note 20.] See also for the blood of the Annelides, Quatrefages, Ann, d, Sc. Nat. XIV 
1850, p. 287. — Ep. 
