forms a compact mass filling the posterior half of the body. Though nearly the whole of
this mass is liver, yet its colour and external appearance are due to the layer of the hermaphrodite
gland which envelopes it. On its upper surface lie the heart in the pericardium,
and the intestine, which, issuing from the anterior portion of the stomach, describes
a bend forward and to the right before running backwards to the anal papilla among the
branchias. On the liver lie also the renal organ and the aorta, but they are not conspicuous
owing to their colourless transparency. The right anterior portion of the body-cavity is
filled with the genitalia. Their bulk 'is very variable according to the condition of the
animal, but in the breeding season they are much dilated and extend under the oesophagus
towards the left.
In a preserved specimen the'intestines are generally of a uniform yellowish colour,
but if a freshly killed Doris be opened much greater diversity will be seen. The
membrane enveloping the viscera has usually a pale purple tinge, deeper in some places
than in others. The buccal mass and oesophagus are yellowish-grey, while the nerve-
collar surrounding the latter is of a deep orange. The stomach varies in colour according
to its contents, but is often brownish-green. The pouch attached to it is generally olive.
The liver is of a deep sage-green, but can hardly be seen as it is almost entirely covered
by the hermaphrodite gland, which is cream-coloured, with very fine red and yellow
markings due to the sympathetic nervous system. The ampulla of the hermaphrodite
gland is dead opaque white and the penial bag translucent white. The spermatotheca is
brown or colourless, according as it is full or empty, and when it is brown the spermato-
cyst is generally yellow. The mucus-gland is pellucid white, but the albumen-gland and
the parts adjoining it are rather bright orange.
A limentary S ystem.
The mouth leads into the oral tube, a short chamber with longitudinally lamellated
walls. Just before the entrance of the main buccal cavity these lamellas are interrupted
by a ring-shaped depression running round the inner wall of the oral tube. The buccal
cavity is entered by the inner mouth, which is a slit in the labial disk. This disk is thick
at the sides, but channelled at the bottom so that when the roof of the buccal cavity is
cut, as is usual in dissection, it falls apart in the form of twQ pads. In D. tuherculata it is
merely covered with a strong cuticle, but in some forms (e. g. Gadlina, Rostanga) it is
strengthened by a ring or plates formed of minute chitinous rods.1 The floor of the
buccal cavity is formed by the odontophore, a tongue-like prominence divided into two
halves by a longitudinal- furrow and formed chiefly of cartilage. It is attached by bands
of muscular fibre to the radula, a membrane covered with small hooks or teeth, which can
be drawn backwards and forwards over the odontophore as over a pulley, and thus tear
to pieces any substance which may be pressed against it. The radula fits into the groove
of the odontophore, being depressed and folded in the middle. There are no teeth on
1 Alder and Hancock commonly speak of this arrangement as a prehensile or spinous collar. But
the expression is misleading, for the armature even when circular is not a collar which fits round
anything, but a thickening on the inside of a tube.
ANATOMY. 39
this median line, but on either side of ifc are from forty to fifty rows, each containing
between fifty and seventy and sometimes as many as ninety teeth. The radula emerges
from a curved sac which hangs down behind the buccal mass and contains special cells
which continually Secrete new teeth. The front of the radula becomes worn by use and
thus the anterior rows of teeth are much damaged, but as they drop off fresh rows emerge
behind from the radula sac and the whole apparatus moves forward. In a large Doris
about twenty rows of teeth are in use on the odontophore and twenty-five more remain in
reserve behind, while six are still in process of formation. The organs mentioned are
often described collectively under the name of the buccal mass, and as seen lying in the
body-cavity present the appearance of a roughly conical bullet. The whole mass is
capable of considerable movement backwards and forwards, and (in some species at any
rate) can be everted. These movements are effected chiefly by three pairs of anterior
retractor muscles attached near the line where the oral tube passes into the buccal
cavity, and a single pair of posterior retractors attached to the lower surface of the
cavity just in front of the radula sac. But probably the animal can control the pressure
exercised upon various organs by the blood which fills the body-cavity and everts the
buccal parts by concentrating the pressure on them.
At its posterior end the buccal cavity passes into the oesophagus, and just at this
point enter the ducts of two salivary glands, one on either side. They are long bandlike
structures traversed by a central canal which pours its secretion into the buccal
cavity near the root of the tongue.
The oesophagus is a fairly broad tube, which may be an inch long, with thin walls
bearing internally'not very conspicuous longitudinal lamellae. It runs straight to the
stomach and under it, entering it posteriorly on the lower surface, whereas the intestine
issues from the stomach anteriorly and on the upper surface. This arrangement, which
seems most unnatural in a symmetrically disposed animal, is clearly explicable by the
larval structure in which the alimentary canal was U-shaped, the mouth and anus
forming the ends of the two limbs, and the stomach the base of the U, but lying rather to
the right. When this arrangement was replaced by longitudinal symmetry, in which the
mouth remained anterior and the anus became posterior, the front part of the right limb
of the U was bent round and prolonged backwards, but the U was not pulled into a
straight line, and the direction which the intestine even now takes in starting still
indicates it§ original destination.
The stomach is a large bag lying in an anterior cleft iof the liver, rather to the left of
the median line. Its walls are thin, more or less lamellated1 but not at all muscular, and
take their colour from the contents, which often render the organ conspicuous among the
other viscera, since, when distended with fragments of red or dark sponge it appears as a
red or black mass. Into the posterior part open four or five liver-ducts2 and also a pear-
1 This lamellation varies greatly in different individuals. Sometimes the walls are almost
entirely smooth.
3 From different points of view it is equally correct to say that there are four or five liver-ducts,
or that there is only one. The stomach, which is a roughly conical bag, communicates with the liver
at its narrow end. If this narrow end is regarded as a tube we may say that the various liver-ducts
unite in one duct before entering the stomach. But its tubular character is not very distinct, and
we may equally well say that four or five liver-ducts open into the narrow end of the stomach.