NATURAL HISTORY OF THE GENUS DERO. 
93 
better course open than to entirely reject it. Up to the present 
time the name is occasionally used on the Continent ; but as the 
worm designated may be any one of three species at least, and as 
no clear definite description has yet been given of any of these in 
particular under the name in question, this cannot be used as an 
argument against the course proposed. 
In acknowledgment of Muller’s work, I have attached his own 
name to the last new species discovered. 
The name by which the genus is now known was bestowed upon 
it by Oken (20) in 1815, and the species described by Eosel dis- 
tinguished as Dero furcata , Muller’s appellation being retained 
for the other species. This was apparently the extent of Oken’s 
knowledge of the genus, as his figure was taken from Muller, 
and his diagnosis is as scanty as possible. 
Dutrochet (9) rediscovered the genus, and being ignorant of 
the work of earlier writers, renamed it Xantlio, remarking that it 
appeared either not to have been observed before or to have 
been confounded with JXais. He also described the branchial 
funnel as an organ of progression and retrogression and of pre- 
hension ; the branchial processes being spoken of as veritable 
non-articulated limbs, resembling the feet of cephalopodous 
Mollusca and the arms of the polypis ! The number of branchial 
processes is given as ten in one species and six in another, 
to which the names Xantlio decapoda and X. hexapoda are respec- 
tively given. As the figures which accompanied the original 
paper appear to have been lost, any attempt to identify Dutro- 
chet’s species must be mere guesswork. 
Blainville (1) gave a somewhat unintelligible account of two 
species, named Nais digitata and N. decapoda. The former is 
credited with the possession of six pairs of fleshy lobes, the 
latter with five, reference being also made to a drawing in the 
£ Encyclopedic Methodique ’ of a species with no less than eight 
pairs. Having had an opportunity of examining the latter work, 
I found that the plate in question is merely a replica of Muller’s 
(18) ; while the peculiarly unmethodical character of the arrange- 
ment of the text renders any search for the article referring to 
the figures hopeless. However, as none of Muller’s figures show 
more than four pairs of lobes, Blainville has evidently interpreted 
them wrongly. 
Blainville (2) in a second article in the ‘ Dictionnaire des Sciences 
natureiles ’ and a writer in ‘ Rees’s Cyclopaedia ’ (23) quoted from 
LINN. JOURN. ZOOLOGY, VOL. XX. 8 
