^ 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 YII. 



KESPIRATORY 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 extremity 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. 



'is Arenicola ; see Milne Edwards, \oc. cit. PI. Micrura fasciolata {Ehrenberg, Symb. phys. 



Xni. PJiytozoa Tab. IV. fig. 4. e. i. g.). 



19 Eunice ; Ibid. PI. XII. fig. 2. s See Quatrefrges, R^gne anim. illustr. Zooph. 



20 With Eunice, Nephtys,Glycera, and Areni- PI. XXXIV. fig. 1, b. b. {Nemertes Camillae). 

 cola, the blood is red ; with Phyllodoce, it is yel- 4 Rathke (see above, § 149, note 1) is of the opin- 

 low ; and it is nearly colorless with Aphrodite, ion that these two cephalic fossae are tlie seat of 

 Polynoe, and Sigalion ; see Milne Edwards, loc. touch ; but the view of Orsted (Beschreib, d. Platt- 

 cit. p. 196.* Würmer, p. 18, 77), who thinks them of a respira- 



1 See Müller, Zool. Danica. Tab. LX V 111. fig. tory nature, is, perhaps, the more correct. In sup- 



1-4 (Tetrastemma (Planaria) viride) ; Delle port of this last, is the fact of the presence of cili- 



Chiaje, Memor. loc. cit. Tav. LXXVIII. tig. 8, a ated epithelium, and of a very large blood-vessel 



(Polia geniculata) ; Quoy and Gaimard, Atia.3 directly beneath them (see Quatrefages, \oc. cit. 



Zool. de 1' Astrolabe Zooph. PI. XXIV. fig. 10 (^Bor- PI. XXXIV. fig. 1, g. g. (Nemertes Camillae) ) 



/asj'a Djridw) ^ and the Diet. d. Sc. Nat. LVII. Art. and which, in many Nemertini, is clearly seen 



P'ers, p. 574, PI. Parentomozoaires, Nemertes, fig. tlu-ough the thin epithelium ; see Midler, Zool. 



1, 2 (Borlasia Angliae, and Cerebratulus biline- Dan. Tab. LXVUI. {Tetrastemma vivire) ; Delle 



atus) ; also Huschke, Isis, 1830, Taf. VU. fig. 1-3. Chiaje, Memor. Tav. LX.XVIII. fig. 8 {Polia 



Xfotospermus drepanensis. geniculata), and Isis, 1830, Taf. VII. {Notosper- 



n Tetrastemnta viiide, Polia geniculata, and mus drepanensis). 



* [ § 157, note 20.] See also for the blood of the Annelides, Quatrefages, Ann. d. Sc. Nat. XIV 

 1850, p. 287. —Ed. 



