marine British Shelis and Animals. 199 
line on each side of the back; between these lines, the dorsal 
ridge and the peduncles are _— with the same: vent on the 
right side. 
Hanah scarcely a quarter of an inch, Amongst fuci, on the 
coast of Devon: rare. 
This elegant little animal would iain be considered by 
the French naturalists to belong to the same genus as the last ; 
but the tentacula not appearing to be retractile, and its being 
destitute of anal plumes on the back, together with the disposi- 
tion of the lateral appendages, lead me to consider it as consti- 
tuting a link between the Tritonia and the Scyllea. 
It has been often a matter of wonder, why these and many 
other similar aquatic Vermes should be furnished with such rami- 
fied or fasciculate appendages, which, to a common observer, 
seem to be destitute of use ; but, by the assistance of modern ` 
philosophy, we are led to believe that they are of such essential 
- service as to constitute the principal agent of vital action, being 
to them what pulmonary organs are to terrestrial warm-blooded 
. animals and some others, but peculiarly constructed for the se- 
paration of oxygen gas or vital air, from the medium in which 
they reside; and thus, like the gills or respirative organs in fishes, 
constituting their principal branchiz or breathing apparatus. - 
SpIo CRENATICORNIS. 2 
| Tas. XIV. Fig. 3. a. 
Spio filicornis. Gmel. Syst. vi. p. 3110? 
Body slender, much resembling that of a Nereis, tapering a - 
little, and furnished with about sixty joints, terminating poste- 
riorly with two short styles; the joints are furnished with pedun- 
cles and fasciculi; upon the upper part of the former are long 
cirri standing erect, with their points usually reflecting over the 
back, 
