very readily fall off, and possibly may be detached, as Professor E. Forbes supposes, at the
will of the animal, but of this we have not been able to satisfy ourselves. Their deciduous
character has been thought by some to offer, a decided objection to their being considered as
branchiae, and indeed the animal is quite capable of surviving the loss of the whole of these
processes; but as it is now very well ascertained that the mollusca of this order respire also
by the skin, even in cases where the branchial organs are more decidedly speciallized, we
see no good reason for withholding the name of branchiae from these organs, though they may
perform that function less perfectly than the processes of some other genera of the order.
The foot is linear, and generally rather narrow. The anus is dorsal and lateral, forming a
small nipple on the right side between the first and second branchial processes. The aperture
of the generative organs is placed below the first branchial process on the right side.
The anatomy of Doto, notwithstanding its general similarity to that of Eolis, differs
from it in some interesting particulars.
The digestive system has the buccal mass much smaller than usual, of an oval form,
and unprovided with corneous jaws. The tongue is curved and placed in the mouth as in
Eolis; it is stiff, slender, and exceedingly small, and is composed of upwards of a hundred
transverse plates each supporting a central tooth, directed backwards, and appearing, when
highly magnified, a little denticulated at the sides. From the small size and weak armature
of the mouth, compared with other genera, we think it likely that the animals of this genus
feed only on the soft parts of the corallines, on which they are generally found. The salivary
gland is ample. It lies above the generative organs on the left side of the body, and
extends forward to the buccal mass, opening by a slender duct immediately before it through
the inferior wall of the channel of the mouth. There is likewise another gland connected
with the channel of the mouth which is probably accessory to the above, though it differs
from it in structure. It is much smaller, and is composed of numerous minute oval sacs,
each furnished with a delicate and suddenly constricted duct: these sacs surround the
channel of the mouth, and pour the secretion into it on all sides just where it receives the
duct from the large salivary gland. With a high magnifying power they appear to be
covered over the inner surface with nucleated cells, having minute vessels passing from
them, which ramify over the inner surface of the sacs and unite as they descend towards
the ducts. The size of the vessels corresponds exactly with the diameter of the nucleus.*
The oesophagus is long, very slender, and passes from the upper posterior dorsal aspect
of the buccal mass, increasing a little in size as it reaches the stomach, which is small
and elongated. The'interior is minutely granulated, but does not appear to be raised into
folds or wrinkles. The intestine is very short and wide, especially at its junction with the
stomach, which takes place further forward than usual on the right side: the inner surface
is plicated longitudinally; the plicae end abruptly as they reach the stomach. From this
peculiarity of structure, and the great width of the intestine, it seems probable that it may
have some other function to fulfil than that of conveying off the refuse of digestion. May it
not be in this canal that the nutritive portion of the food is absorbed into the digestive
* We have been thus minute in the description of this gland, as we think it likely to throw some
fight on the ultimate structure of glands in general. We should suggest the probability that the
nucleus of the cell of glandular tissue may be nothing more than the orifice of a minute vessel, or
perhaps, in some cases, a mere opening through which the secretion passes.
system ? The anus opens on the right side of the back, as stated above. Two delicate
vessels or ducts pass from-the anterior portion of the stomach to the first pair of branchial
processes: the other branchial processes receive similar vessels from a central vessel which
passes from the posterior extremity of the stomach, and extends the whole length of the
body beneath the ovarium. The lateral vessels alternate a little, and are not always symmetrical
: the last pair arise from the extreme posterior point of the central vessel.
The hepatic glands are very highly organized, and almost fill the branchial processes,
with the exception of small passages hollowed out of the glandular mass, probably for the
circulation of the blood. There are no distinct sheaths as in Eolis, though we have observed
between the gland and the outer skin in JD. coronata, a sort of network of fleshy columns
similar to what we have seen between the outer and inner sheaths of the papillae of Eolis
papillosa. On placing one of the smaller branchial processes of JD. fr a g ilis in the compressor
of the microscope, the gland is discovered to be of a highly racemose character:
but for the complete understanding of its structure it is necessary to make a transverse
section of one of the larger processes, when the gland is perceived to be highly complicated
and delicately branched. The branches radiate from a minute central duct, and are much
divided and crowded with follicles: their extremities reach into the papillae. The completeness
of this organ as a secreting apparatus must be evident to physiologists, as well as
the improbability of its subserving any other function.
Respiration is undoubtedly performed by the surface of the body as well as by the
branchial processes. Immediately beneath the skin of the body in JD. coronata, is the same
network of fleshy columns observed in the branchial processes of that species; and both in it
and in JD. fra g ilis , the entire surface is covered with vigorous vibratile cilia. Cilia are also
on the branchial processes, but are most vigorous towards their bases.
The heart is as completely organized as in any of the order: the ventricle is strong
and muscular, the auricle thin and membranous.
The nervous system is furnished with four cerebral ganglions of nearly equal size, and
two small buccal ganglions of the usual form. The nerves are arranged much as in Eolis.
There appear to be about ten principal pairs, and a few smaller ones, which we were unable
to trace. The olfactory ganglions are at some distance from the cerebral ganglions to which
they are united by large nerves : they are round, and placed at the bases of the tentacles.
The eyes are also placed a little way from the cerebral ganglions, and are very small. The
otolites of the auditory capsule are numerous and elliptical.
The generative organs do not materially vary from those of Eolis. The ovarium, however,
is above the great central vessel of the digestive system.
EXPLANATION OF THE PLATE.
Fig. 1. General view of the viscera of Doto fragilis, seen from above:—a, buccal mass; b,
oesophagus; c, stomach; d, intestine; e, anal nipple; f } salivary gland; g, accessory
salivary gland; h, h, h, h, lateral vessels or ducts leading from the branchial glands to
the central vessel connected with the stomach; i, ventricle of the heart; j , auricle of
the same; k, k, k, generative organs; l, cerebral ganglions; m, m, olfactory ganglions.
2. The digestive apparatus exposed :■—a, buccal mass; b, oesophagus; c, stomach; d, intestine
; e, anus; f f great central duct; g, g, g, g, lateral ducts from the branchial
glands; h, salivary gland; i, accessory salivary gland.