G e n u s 11. DENDRONOTUS,* A l d e r a n d H a n c o c k .
• Corpus elongatum, lateribus compressis, ssepissime lato altus: pallio nullo. Caput subinferius,
velo parvo, ramoso, obtectum: xnaxilbs corneis. Tentacula duo dorsalia, clavata et laminata, intra
vaginas retractilia. Branchial, cylindrical ramosse, linea unica utrinque dorsi dispositae. Pes linearis,
gracilis. Orificia generationis et ani ad latus dextrum.
We have found it necessary to institute this genus for the well known Tritonia
arborescens of authors and its allies, which are distinguished from the true Tritonia
(T . Hombergii, &c.) by the form of their tentacles, the want of a sub-pallial ridge on the
sides of the back, and the free arborescent nature of their branchiae. These characters
alone had induced us to consider them generically distinct before we had an opportunity of
examining their internal structure; but it was not until we had dissected each that we were
aware of the full extent of their differences; the gastric system of Dendronotus presenting
the ramifications so peculiar to the family Eolididce, while the stomach of Tritonia consists
only of a simple pouch without appendages, as in the Doridida; the liver in the former
instance being much broken up and occupying the sides of the ramifications, whilst in the
latter it forms a single mass in the posterior part of the body. This important character
obliges us not'only to form of the species so differing anew genus, but to remove them into
the family Jjhlidida, to which, from their structure, they properly belong. Even in an
anatomical point of view, however, the genus Dendronotus will be found to have only
partially assumed the characters of > this family, retaining still much resemblance to the
Tritoniadte, and constituting one of those transition forms so often found uniting the
different types of organization in the animal kingdom.
The body is elongated, much compressed at the sides, and tapering to a point behind.
The back is rounded, and has no vestige of a carinated ridge at the sides, or rudimentary
cloak, as is observable in Tritonia. The veil is short and more or less branched, covering
the head, which is sub-inferior and indistinct. There are two tentacles, which are dorsal,
clavate, and transversely laminated on the upper part as in Doris: they are retractile
within sheaths, generally branched at the margins. The branchiae are arranged in a single
line on each side of the back, rising gradually from it at their base, and dividing into
cylindrical branches much resembling a tree without leaves, or, perhaps more closely, a
branch of coral. This structure is different from that of Tritonia, whose branchiae are composed
of flattened leaflets or plumes like those of the Dorididee. We have not been able to
detect the ciliary movements on these organs, though we have found cilia in vigorous action
along the margin of the foot and on the anal nipple : it is therefore likely that they exist
over the whole of the body; and that they cover both the branchial tufts .and branches of
Prom SevSpov, a tree, and vojtoq} the back.