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XXI. On a new Species of British Annelides belonging to the Family Cheetopteridee. 
By W. BAIRD, M.D., F.L.S., $c. 
(Plate XLIX.) 
Read April 21st, 1864. 
AMONGST the Dorsibranchiate Annelides there is a peculiar genus, the position of 
which, in the arrangement of these animals, has been much misunderstood. It was 
first instituted by Cuvier in 1830, in the second edition of his * Règne Animal,’ for the 
reception of an anomalous-looking worm which lived in a kind of leathery case, and 
inhabited the seas of the West Indies. This genus he named Chetopterus*, from the 
peculiar structure of the pedal organs, assuming as they did greater proportions than 
any' of the others, projecting laterally and extending like a wing on each side of the body, 
and being supported by a series of bristles. Cuvier was exceedingly at a loss where to 
place the genus, and accordingly assigned it a “locus peenitentice,” as it were, at the end 
of his Dorsibranchiata. The details, with regard to its characters and the organization 
of the only species then known, he left to be given by MM .Audouin and Milne-Edwards. 
These authors accordingly furnished a more lengthened description of the genus in 
the Ann. des Sc. Nat. for 1833; but, though they state that it is difficult to arrange 
this peculiar genus amongst the Dorsibranchiate Annelides (—their order Errantes), and 
though they think that it would be * more natural" to form a distinet order for its 
reception, they leave it in the order in which Cuvier placed it, only forming for it a 
family apart from all others. This family has been adopted by all succeeding authors; 
but the place which was assigned to it, viz. between the peculiar family Peripatide and 
the Arenicolide, is evidently erroneous, as with neither of these has it much affinity. 
Leuckart, in a paper upon the genus CA«etopterus, in Wiegmann's ‘Archiv’ for 1849, took 
à more correct view of its affinities. He expressed an opinion that it undoubtedly ought 
to belong to the large family Ariciide, an opinion which is evidently correct, as we will 
show more fully hereafter, Grube, in 1850, either not knowing this paper or dissatisfied 
with the author’s conclusions, arranged the Cheetopteride amongst the Limivora or 
mud-eating Annelides, and assigned as their place in the system a position between his 
family Mo consisting of the single genus Siphonostomum, and the family Telethusa, 
composed of the 4renicole or ** Lug-worms" of fishermen. Sars, in his ‘ Fauna littoralis 
Norvegiæ, published in 1856, described a new species belonging to the family Cheetopte- 
ride, found on the coast of Norway, and, adopting the view taken by Leuckart, discovers 
in this animal a still greater resemblance which some of the Ariciidw bear to the Chze- 
lopteridze. This is particularly the case with the genus Spio, which had always been 
arranged by systematists amongst the Ariciide, a resemblance which induced him to 
give to his new worm the generic name of Spiochetopterus. 
* From xaírz, a bristle; and rrepov, a wing. 
VOL. XXIV. : : 3Q 
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