Genus 8. IDALIA,* Leuckart.
Corpus ovatum, crassum, antice declive, postice- acuminatum, vix palliatum. Caput subinferius,
subincrassatum, non tentaculatum. Tentacultt 2 dorsalia, linear!a, lamellata, non-retractilia. Area
dorsalis cirris marginata. Bronchia plumosae, in lined, media dorsi annm cingentes. Apertura
genitalis ad latus dextrum.
Idalia is a well-marked genus, characterised by Leuckart in 1828, in a small publication
entitled c Breves Animalium quorundam maxima ex parte marinorum Descriptiones.’t
The genus is- distinguished from Triopa and JEuplocamm, with which Dr. Philippi has
confounded it, by its less destinct cloak, which forms a small dorsal area, not extending
beyond the tentacles in front, and leaving a considerable space between it and the frontal
margin of the head: the tentacles too are without sheaths, and the filaments which adorn the
pallial margin are generally very long in front of them. These characters likewise distinguish
it from the other allied genera.
The prevailing colour in Idalia is red. Its range appears to be pretty extensive, and,*
though nowhere common, members of the genus are found in most of the European seas.
They chiefly live in deep water, and hence, perhaps, the infrequency of their occurrence.
They are carnivorous.
The body in this genus is ovate, thick, and a good deal elevated on the central area of
the back, from which it slopes down on all sides, especially in the frontal region, which is
large and incrassated. The head is indistinct, broad, and subinferior, with a slight expanded
margin above. There are no oral tentacles. The two dorsal tentacles are generally slender
and linear: they are laminated,—sometimes only slightly and on the posterior side. The
base of each is adorned with one or two filaments, which have sometimes been taken for
additional tentacles. These are not attached to the tentacles themselves, as in Ancula, but to
a thin, and rather inconspicuous membrane—the margin of the rudimentary cloak—that
circumscribes the dorsal area, passing close to the base of the tentacles in front, and running
along each side of the back to the branchiae. This margin is more or less fringed with
filaments; the inclosed area has also frequently one or more rows of filaments. The branchiae
are plumose, and surround the vent on the posterior dorsal line: behind these a ridge runs
down to the tail. The foot is broad, thick, and rounded in front. The aperture of the
generative system is on the right side.
Idalia may be divided into two sections, differing from each other in the armature of the
month, as well as in the following external character.
Section T; Centre of the hack with filaments. Type, I. elegans.
Section 2. Centre of the back without filaments. Type, I. aspersa.
Our observations on the internal organisation of this genus will have to be confined to
that of I. Leachii, as this is the only species we have had the advantage of dissecting, and
we are not aware that any description of the anatomy of the other forms has been
published. We have, however, examined the lingual apparatus of four or five kinds.
* A name of Venus, from Mount Idalium, in the Isle of Cyprus, sacred to that goddess.
f We are indebted, for a copy of this publication, to Dr. Riippell, of Frankfort.