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, ot 
various forms; sometimes divided like a comb, or branched like a 
tree, sometimes composed of simple filiform appendages resembling 
the cirré of the oars. In other ringed-worms the gills are situated 
at the most anterior part of the body. In the Leech, the Harth- 
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 
(none and Aglaura Say. 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 cesophagus, is straight!; GRUBE observed the same dis- 
position in Cirratulus?. In Amphictene (Amphitrite auricoma bel- 
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®. In 
the remainder, where the canal is straight, there are usually lateral 
appendages, or it is as though divided into cells by transverse 
constrictions. In the Earth-worm a short round stomach succeeds 
to the cesophagus, 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 Isis, 
1832, s. 6547, Tab. X. fig. 13. 
2 A. E. Gruse, Zur Anatomie und Physiologie der Kiemenwiirmer, Konigsberg, 
1838, 4to. 8. 34. 
3 PaLLas, Misc. Zoolog. p. 129, Tab. 1X. figs. 12, 13. 
