210 CLASS VII. 



base, or foot-piece of each of these oars there is usually placed 

 a filiform appendage (cirrus). In the Dorsibranchiates there are 

 found, in addition, on the dorsal surface towards the sides and near 

 the oars, or upon them, the external respiratory organs, Gills, of 

 various forms ; sometimes divided like a comb, or branched like a 

 tree, sometimes composed of simple filiform appendages resembling 

 the cirri of the oars. In other ringed-worms the gills are situated 

 at the most anterior part of the body. In the Leech, the Earth- 

 worm, and allied genera, no respiratory organs are visible externally. 



In those ringed-worms that have not a distinct head, the mouth 

 is usually found quite at the anterior extremity of the body ; in the 

 rest it is situated on the* inferior surface, and usually a muscular 

 proboscis can be everted (Phyllodoce Nereis, &c.). In these, more- 

 over, the mouth is ordinarily armed with horny jaws, placed late- 

 rally, differing in number in the different genera. Occasionally the 

 number is not the same on the two opposite sides. Thus the genera 

 (Enone and Aglaura SAV. have four jaws on the right, five on the 

 left ; Lysidice and Leonice three on the right, and four on the left. 



The intestinal canal is, for the most part, straight, yet there are 

 exceptions. In Sabella ventilabrum the canal makes a great num- 

 ber of transverse flexures, lying upon one another, and winding 

 sometimes to the right, sometimes to the left : the first portion 

 alone, the oesophagus, is straight 1 ; GRUBE observed the same dis- 

 position in Cirratulus 2 . In Amphictene (Amphitrite auricoma bet- 

 gica Cuv.) the intestinal canal makes two flexures, as in Holothuria, 

 first proceeding backwards, then straight forwards, and then back- 

 wards again with a narrower portion between the two others 3 . In 

 the remainder, where the canal is straight, there are usiially lateral 

 appendages, or it is as though divided into cells by transverse 

 constrictions. In the Earth-worm a short round stomach succeeds 

 to the oesophagus, and then there is another muscular stomach. In 

 Arenicola the middle wider portion of the intestinal canal has very 

 thin walls, and is covered with very regular vascular reticulations 



1 MECKEL, System der Vergl. Anat. iv. 1829, s. 71, R. WAGNER in OKEN'S 7*w, 

 1832, s. 657, Tab. x. fig. 13. 



2 A. E. GRUBE, Zur Anatomic und Physiologic der Kicmcmwirincr, Konigsbcrg, 

 1838, 4to. s. 34. 



3 PALLAS, Misc. Zoolog. p. 129, Tab. ix. figs. \i, 13. 



