The oesophagus is short, but usually found bent in preserved specimens, that is to
say it can be straightened so as to allow of the protrusion of the whole buccal mass. In
this species the salivary glands are small, and their ducts which are inserted behind
the buccal bulb are long, thin, and inconspicuous. They lead to minute racemose glands
attached to the inner side of the dorsal integuments. In most JEolids these organs are
long bands, and besides the pair of salivary or post-bulbar glands there are usually
ptyaline or prebulbar glands set on the wall of the oral tube in front of the buccal bulb.1
The stomach is median in position and somewhat elongate. Both it and the oesophagus
are lamellated internally. The intestine issues from its right side, and describes a very
short curve before running backwards and terminating in the anal papilla. I t is broad
and has relatively thick walls which are lamellated on the inner side and contain fine
muscular fibres, crossing one another in all directions. At the point where it leaves the
stomach there begins a ridge which at first runs along its inner dorsal wall (appearing externally
as a series of puckers) but tends downwards until it reaches the sides or floor of
the tube. In the region of the stomach the ridge is merely furrowed and the intestine
finely lamellated, but in their further course both the ridge and the walls of the intestine
develop numerous and conspicuous folds which almost fill the lumen of the tube, and
apparently act as a sphincter.
Into the stomach open three principal liver-ducts, one on either side and one posterior
which is the largest of the three and runs backward along the median line of the
body-cavity above the hermaphrodite gland. I t might also be described as a posterior
prolongation of the stomach. The liver itself lies outside the main body-cavity and is
contained within the cerata in the form of lobes, which are flattened and roughly conical
in shape, but bear on the surface small follieulate projections. Within each _lobe is a
central cavity with side pouches corresponding to the projections,. This cavity is
continued into a tube which passes through the body-wall and unites with other tubes
until they all enter the stomach by one or other of the three main ducts already mentioned.
I t has been proved by experiment that food passes from the stomach into the liver-lobes
within the cerata, the passage being facilitated by the alternate contraction and expansion
of the stomach and main ducts visible in living animals.
In some aeolidiform animals the liver is found partly free within the body-cavity and
partly enclosed in the cerata, and even when there is no free portion, the tubes which
are ramified within the dorsal integuments or which traverse the body-cavity may be lined
with coloured hepatic cells (e. g. in Coryphella o'ufibranchialis and Tergipes despectus).
But in jEolidia papillosa the hepatic cells, though varying slightly in distribution in
different specimens, never extend far below the bases of the cerata. Below each
transverse row of cerata runs a tube into which their bases open, and two or more of' such
tubes unite to form a broader but shorter tube which enters one of the three main ducts
mentioned above. On looking at the arrangement in another way, we may say that the
main posterior duct gives rise to six to seven secondary ducts on either side, not set quite
1 Alder and Hancock do not seem to have found in ,/E. papillosa the glands mentioned above,
and they give the name of salivary to two small glands in the cheeks of the buccal mass. I t must be
remembered that these designations, salivary, ptyaline, etc., are mere names, and that nothing is known
as to the action of the various secretions which they pour into the buccal cavity.
evenly in pairs or opposite one another; these bifurcate once or twice and the cerata
communicate with the tubes formed by the bifurcations. The two anterior main ducts
are essentially similar to the posterior duct but less extensive.1 The main ducts have
slightly lamellated walls, like those of the stomach but thinner, and are sometimes
distended by chunks of food awaiting disintegration and digestion.
As already mentioned, the alimentary system of most ^Eolids also acts by a most
unusual adaptation of its functions as a defensive apparatus. The urticating cells or
nematocysts of sea-anemones and other Ccelenterates which the animal eats pass into its
stomach and thence to the cerata, where they are excreted by a special process and stored
in chambers ready for use. These chambers or cnidosacs are situated at the top of the
cerata and can generally be seen clearly through the integuments with (or even without)
the aid of a lens.® They communicate by very narrow canals with the hepatic lobes
which terminate about a millimetre below the tip of the cerata so as to leave room for
the cnidosacs. The canal of communication is bent upon itself and is lined with very
long cilia, which appear to have the power of stopping the entrance of all bodies
except the nematocysts. The cnidosac is lined with special cells (the cellules nSmato-
pliages of Cu6not) shaped like truncated pyramids.3 These cells appear to attract and
ingest nematocysts, which on passing into them almost always assume a position in which
the closed end points to the base of a cell and the invaginated end towards its free
extremity. After a cell has ingested several nematocysts it undergoes degeneration and
becomes merely a membranous bag containing a store of arms. The nematocysts are of
various shapes, and as they are derived from the Coelenterates on which the ^Eolid feeds, a
particular shape is not necessarily characteristic of a particular species of ^Eolid. But
the essential structure of all is the same. They are minute capsules filled with a transparent
liquid. One end is closed and the other end is invaginated, the invagination taking
the form of a tube. The details of the process of explosion are not quite clear4 but the
result is certain. When a nematocyst has been discharged the capsule remains, but at
the end where there was formerly only an invagination there is now a barbed external
process which is usually terminated by>a long filament and is capable of penetrating and
wounding delicate tissues. It does not appear that an AEolid has any power of causing
its cnidosacs to discharge nematocysts spontaneously, but when the cerata are bitten or
otherwise squeezed (which is of course a violent convulsion for a delicate organ like the
cnidosac), the cells are ruptured and the nematocysts discharge as they escape.
N ervous S ystem.
The central nervous system of -dSolids is in its essential features similar to that of
\ ’ The whole arrangement is well illustrated in the Monograph, Fam. 3, PI. 7, figs. 1—4.
2 Though 2E. papillosa offers many advantages to the dissector on account of its great size, its
integuments are less pellucid than those of other species.
3 There are also between them smaller interstitial cells which do not ingest nematocysts.
4 See Cuenot and G-rosvenor. It is suggested that the eversion of the barbs and thread is due to
osmosis and the sudden entrance of water into the capsule. It is supposed that the contents of the
.capsule are hygroscopic, and, though unable to absorb the concentrated solution which surrounds them
in the tissues of Ccelenterates or ASolids, offer a passage to sea-water.