V I .
ANATOMY.
T he anatomical descriptions scattered through the works of Alder and Hancock are
believed to be due almost entirely to the latter author, who also wrote on these questions
both in conjunction with Embleton and in his own name only. They are of such
excellence that they still furnish for many purposes the best and clearest account of the
internal organs of the Nudibranchiata. But naturally the prolonged and detailed
researches of Bergh, Trinchese, Vayssiere, and other investigators, have collected many
new facts respecting the structure and functions of these organs, and also, it must be
confessed, that lucid as are the anatomical sections of the Monograph, the authors
frequently employ a terminology which is not only antiquated but erroneous. This is not
a mere matter of words; when for instance Alder and Hancock call the hermaphrodite
gland the ovary, a wrong (or at least an incomplete) view of its functions and even of its
structure is implied. But a detailed criticism of their accounts would not be very
readable or profitable, and it will perhaps prove more useful to write fresh anatomical
descriptions of Doris talerculata and JEolidia papillosa. The repetition which this
entails is, I hope, justifiable, for it is so difficult to find language which gives a clear idea
of the structure of invertebrates,.that a re-statement~of known facts in new words is
often of assistance.
I have selected the two animals mentioned because they are of considerable size and
are common on our coasts; they also represent in their most usual and highly developed
form the types of structure characteristic of the Holo- and Cladohepatica respectively.
In speaking of them as types it is not, of course, implied that all the members of the two
classes are constructed on the same plan as Doris and JEolidia. On the contrary there is
considerable variety of structure, as will be made plain in the chapter on classification.
But of the Nudibranchs hitherto described probably two thirds are either Dorids or
AEolids, and these two classes certainly exhibit the two schemes of construction which
have proved most successful for shell-less marine molluscs. In details, however, it
cannot be assumed that all Dorids ;are like D. tuberculata, and still less that all AEolids
are like 2E. papillosa. Comparatively few forms, especially in the tropical genera, have
been thoroughly examined, but we know that general uniformity of structure throughout
a class is not incompatible with occasional unexpected variation in particular points.
1. DORIS TUBERCULATA.
Doris tuberculata is roughly oval and is shaped much like a half-lemon. On the back,
which is covered with flat tubercles of various sizes, are two openings for the rhinophores
in the front p a rt: in the median line but near the end of the body is a larger single opening
for the branchiae. "Within the branchial circuit are also the anal and renal papillae. The
dorsal surface is produced so as to form a free flap, or mantle-margin, all round the body.
The ventral surface is formed by a broad creeping sole commonly called the foot. Its
margin is somewhat expanded, and in the furrow formed between it .and the mantle-
margin there lie in front the mouth and on the right side near the anterior end the orifices
of the genitalia. On either side of the mouth is an oral tentacle, which in this species
takes the form of a thick knob marked with a groove. In other Dorids these tentacles
are thin and conical. When the genitalia are contracted the orifices appear as a simple
pit, but when they are extended it is seen that three orifices open into this pit. The
male orifice is the most anterior; behind it is the channel through which fertilization
takes place; below this and slightly behind it is the orifice of the oviducts
The dorsal integuments are fairly thick and contain numerous short spicules slightly
bent in the middle. These spicules are relatively much larger in the young animals and
are to some extent absorbed in the adult, leaving cavities to mark their places. At the
sides the body-walls are thick but spongy, being full of blood-lacunas. The foot consists of
a spongy and of a muscular layer.
The internal organs of Doris fall into five principal divisions :
(1) The alimentary system, consisting of the buccal mass, oesophagus, stomach, liver,
and intestine. These organs are arranged in rough but not complete symmetry along the
main longitudinal axis of the body, from the mouth in front to the termination of the
intestine among the branchial plumes near the end of the body.
(2) The nervous system and sense-organs. The former consists chiefly, but not
entirely, of a collar of ganglia set round the oesophagus ; the latter of rhinophores, eyes,
and otocysts, a pair of each.
(8) The respiratory and circulatory systems consist of the branchiae, the heart, the
blood-vessels, blood-spaces, and the blood-gland.
(4) The renal system is a ramified organ lying on the top of the liver and communicating
with the pericardium.
(5) The reproductive system is extremely complicated and comprises three parts : (a)
The hermaphrodite part consisting of a gland spread over the liver in the form of a thin
layer, which finally discharges its products into a tube which bifurcates; (b) the anterior
branch of this tube is the male portion, which is not further divided; (c) the other branch
is the female portions which is again divided into the vaginal section, bearing two spermato-
thecas, and the uterine section, which is connected with two large glands that secrete the
various envelopes enclosing the egg.
If the back of Doris be taken off like a lid by an incision under the mantle-edge
extending round the body, the intestine and membranes below the branchim being cut,
most of the above-mentioned organs will be seen lying as if packed in a box.1 Immediately
behind the mouth is the buccal bulb containing the masticating apparatus.
From it a tube, the oesophagus, which is surrounded by the nerve-collar, runs into and to
some extent under the stomach. This organ lies on the anterior end of the liver, which
1 See Monograph, Plate 1, Fam. 1, fig. 2. The plates in the Monograph marked Fain. 1, pi. 1, and
Fain. 1, pi. 2, may be consulted for illustrations of the organs described in this chapter, but it should
be noted that many figures are not of Doris tuberculata.