Monstrosities, or sports of nature, are occasionally found amongst the Nudibranchiata. Mr.
Peach sent us from Peterhead an JEolis papillosa, in which there was only one dorsal tentacle,
placed in the centre of the head. Sir J. G. Dalyell figures the tentacles of an Eolis, which he
calls E. monoceros, from the circumstance of its having a single forked dorsal tentacle.’ “A
pillar,5’ he says, “ rises from the middle of the heck, which diverges into two comicula, with
about seven whorls on each.” It was probably a monstrosity of E. coronata. • In both these
instances, the peculiarity appears to arise from the complete or partial fusion of the two tentacles
into one. The latter author also figures a mal-formation of Dotofragilis (called Tritonia
conifera) with “ a large central branchial tuft on the back between the two branchiae of the
third pair.” We have se^n JEolis rufibranchialis with one of the dorsal tentacles bearing a
lateral branch; and have observed a similar sport of nature in the branchial papillae of E.alba
and several other species. More frequently, however, the variations consist of the suppression
of less important parts, to which some species are more liable than others. Polycera quadri-
lineata is extremely variable in this respect; the velar filaments, the rows of tubercles, and
the branchial appendages being alike subject to variation in this species.
Parasites.**1—1The Nudibfanchs are frequently infested with parasitical Entomostraca,
which are usually buried beneath the skin, the ovigerous vesicles only appearing outside; but
occasionally they penetrate into the abdominal cavity, or live freely on the surface. We have
observed several species of these parasites inhabiting the Dondida and Eolidida.
Tethys is liable to be thus annoyed by a large kind; an individual so affected is figured
in Delle Chiaje’s Memoirs, (copied in Mrs. Gray’s ‘ Figures of Molluscous Animals,’ pi. 210,
fig. 1). A small species is occasionally found on some of the Eolides and on Doto coronata. In
two or three specimens of JEolis rufibranchialis which occurred near Whitley, two irregularly
formed elliptical lobes, the ovigerous sacs of this species, protruded from the skin between the
tentacles. The Eolides were apparently quite healthy, and evinced no suffering, notwithstanding
that the parasites must have been lodged in the vicinity of the cephalic ganglions. The lobes
were very dense, pale flesh-coloured, and exhibited a minute granular appearance. The
second day after their capture the lobes disappeared, and in the course of an hour or two they
were replaced by others, in which the granular appearance was very indistinct. Unfortunately,
we have not been able to examine the body of this animal sufficiently to give a description of
it. Two very curious forms,, however, have occurred in some specimens of Dons pilosa from
Devonshire, to which we have paid more attention. Both these were found within the abdominal
cavity resting upon the liver-mass. One . (PI. 45, figs. 6, 7) is nearly a quarter of an inch long,
with a narrow, linear, soft body, of a white colour, without segments, and having the tail or
abdomen tapering a little, and apparently composed of two or three articulations. This
extremity is always attached to the skin within the branchial circle of the Doris, where
the ovigerous lobes were most likely protruded. These, however, we have not seen; though
in one specimen, what appeared to be the remains of them, were still adhering to the tail.
There are three pairs of legs from the sides of the body, which are long, tapering, soft, and
without articulations; they lie closely embracing the liver-mass. The mouth is situated below
in front, and is apparently without appendages.
We have never found more than one individual of this strange form inhabiting the same
Doris; and with it there were always associated one, two, or three of the other species before
alluded to. This latter (PI. 45, figs. 8,9,) is about one sixteenth of an inch in length, depressed,
with the thoracic shield quite distinct from the abdomen, and composed of only one
segment, of ah irregular oval or sub-quadrilateral form, broadest behind: a projection in front
covers the head. The abdomen is narrow; it tapers a little, and is formed of four or five
segments. The tail is bilobed, each lobe bearing a single stout hair. The mouth is situated
below and has on each side a hook, supported on a flexible base; the head is likewise furnished
with a pair of articulate antennae. There are three pairs of rather feeble thoracic feet, the
last articulation bearing a slender hook.
Whether these two forms are distinct species, which from their great dissimilarity might
be supposed to be the case, or are the opposite sexes of the same species, cannot at present
be determined; but their constant association together would seem to point to the latter
conclusion.
We have taken another interesting species (PI. 45, fig. 10) on Doris tuberculata, adhering
to the branchiae. It is not, however, confined to these organs, but ranges over the whole
surface of the body. It is minute, measuring scarcely one sixteenth, of an inch long, is almost
colourless, depressed, ovate, the broad end forward. The thorax is composed of four segments,
the first being very large; the other three narrow. The abdomen is small, and formed of
four or five segments, the first two being larger than the rest and not very distinctly separated.
The tail is bilobed, each lobe bearing five setae, two of which are very long. There are four
pairs of-feet, three of which belong to the thorax: these are. composed of several joints,
each, excepting the last, bearing a single spine; the last joint is flattened and has on its
inner margin several setae, pn the exterior margin three spines. The fourth pair of feet
are rudimentary; they belong to the first segment of the abdomen, and are formed of one
joint, bearing at its extremity two setae. The antennae are long, slender, and composed of
several setose articulations. Immediately behind these, there are a pair of stout arms or feet,
formed of three joints, the last being furnished with two hooks. The eye is deep rose-
coloured. The ovigerous lobes are yellowish-white, of a cylindrical form with the ends,
rounded; the eggs are large, and not very numerous.
Drawings of the above three forms have been submitted to Dr. Baird, who kindly informs
us that the'last-mentioned species belongs to the genus1 JErgasilus, and he has no
doubt that it is perfectly new and undescribed. The same naturalist also, states that the
small form from Doris pilosa very possibly belongs to the genus Domolachus; but that it
appeared to be imperfectly developed. It is also new to him, and very different from any
species with which he is acquainted.
We have seen another species of JErgasilus. This was taken on Antiopa, cristata from
the south of England, and had deep orange-coloured ovigerous lobes.
These ovigerous vesicles of the parasitic JEntomostraca have sometimes been described
and figured as a part of the animals in which they were found.
Distribution. With the imperfect knowledge of foreign species that we yet possess, it is
scarcely possible to arrive at any satisfactory conclusion concerning the general distribution of
the Nudibranchiata in the different regions of the globe.. So far as the observations of
travellers go, they appear to be pretty generally diffused throughout all seas and in all
climates. The tropical forms are, as usual, larger and more brilliantly coloured than those of