2. iEOLIDIA PAPILLOSA.1
JEolidia, papillosa differs considerably from Doris in appearance. It is roughly triangular
in shape; broadest across the rhinophores and tapering towards the tail. The
rhinophores are smooth columns, not retractile, and devoid both of perfoliations and
sheaths. There are no branchial plumes, but the sides of the dorsal surface bear cerata
which entirely cover its posterior part, leaving exposed in front a triangular space behind
the rhinophores in which rises a more or less prominent swelling containing the pericardium.
The cerata are flattened papillas, each containing a branch of the. liver in the
form of a brownish cylindrical core, visible through the transparent integuments. They
can easily be detached, and the back then shows a series of circular or oval facets,
arranged in about twenty to twenty-five rows. In the middle of each facet is a small
orifice corresponding to the end of the liver branch, and with a microscope two minute
openings for blood-passages can also be seen. The cerata in front and near the tail are
smaller than those in the middle, and also in each row they decrease in size going outwards.
In a large specimen the middle rows contain about fifteen. The anterior and
posterior rows are shorter. The anal papilla lies somewhat to the right of the median
dorsal line between two rows of cerata, usually the tenth and eleventh. The mantle, does
not form a flap as in Doris, but inconspicuous ridges running down each side of the body
and meeting at the tail end. In JE. papillosa the dorsal surface overhangs the sides of the
body more than in most species, but there is no trace of a fold over the head. The external
mouth parts and orifices of the genitalia are much as in Doris, but there are only
two of the latter, not three.
The topography of the anterior body-cavity3 (buccal parts, genital mass, and nerve-
collar) is much as in Doris, but the pericardium lies further forward and the arrangement
of the posterior organs is materially altered by the structure of the liver, which consists,
not of a compact mass, but of a number of lobes or branches lying wholly in the cerata.
As a consequence of this the hermaphrodite gland is not a mere layer on the liver but an
independent organ of considerable bulk. The back cannot be taken off like the lid of a
box as in Doi'is, but is connepted with the alimentary organs in the body-cavity by a
number of tubes which have to be cut before it can be removed. Both the integuments
and the walls of the various organs are more pellucid , than in Doris; spicules are entirely
absent, and the general texture is spongy. In life the epithelium is provided with vibratile
cilia. The skin consists of an outer layer which secretes mucus and an inner muscular
layer, varying greatly in thickness in different parts.
1 Those who are interested in Hancock’s anatomical work are recommended to read the series of
papers entitled On the Anatomy of Eolis, by Hancock and Embleton, which appeared in the Ann.
and Mag. Nat. Hist, for 1845, vol. xv, pp. 1—10 and 77—88; 1848, vol. i, pp. 88—105; and 1849,
vol. iii, pp. 183—202.
9 See for figures illustrating this and other points the plates in the Monograph numbered Earn. 3
•plates 7 and 8, and also the articles referred to in the last note.
A limentary S ystem.
The anterior part of the alimentary canal differs from the same region in Doi'is chiefly
in the presence of a pair of mandibles or jaws. These names, though in common use for
the organs, are somewhat misleading, for the so-called jaws do not masticate but act as
shears which cut off pieces of food to be dealt with by the rest of the alimentary apparatus.
They consist of two roughly elliptical laminse which form the sides of the buccal
mass. The anterior and superior margins are considerably thickened and the latter are
united by a ligament which acts as a hinge. Viewed from the front, the only portion
of the jaws visible is the cutting edge, which is constituted almost entirely by an accessory
process distinct from the main lamina. The margins of the jaws both above and below
are covered with thick muscular pads, but their sides are merely coated with epithelium
both within and without so that the buccal bulb is harder and more angular than in
Doi'is. I t does not lie evenly in the direction of its long axis, and the opening of the
oesophagus is not opposite the mouth. The tongue consists of a high raised prominence,
resembling a wedge projecting from the hinder part of the buccal chamber. It bears only
a single row of broad crescent-shaped teeth on each of which are forty to fifty denticles.
The teeth are continued round the tip of the wedge and extend for a little way on the
lower side. The radula sheath is short and does not project behind as in Doris. The
sides of the buccal chamber are bare for about a third of their extent and are formed
merely by the concave side of the mandibles. The rest of the inner surface is lined by
a thick muscular pad, which lies against but does not adhere to the mandible.
The mouth parts are provided with an elaborate system of muscles which regulate
separately the movements of the whole buccal mass, those of the jaws and those of the
tongue. The whole mass can be drawn backwards or forwards by the action of muscular
fibres inserted into the circular belt at the base of the outer lips. At the other end
the retractors are inserted into the foot, the sides of the body, and the dorsal surface,
but the protractors into the upper and posterior margin of the jaws. On the upper
surfaoe of the jaws are two transverse muscles, a small one before the hinge and a larger
one behind i t ; on the under surface there is an anterior transverse muscle attached to
both jaws, and the inner lip, which is very muscular, acts as a sphincter and brings the
cutting edges together. The posterior part of the buccal mass is formed by two layers of
muscles uniting the oesophagus to the muscles of the jaws and of the tongue. The wedge-
shaped tongue is itself a mass of muscular fibres among which can be distinguished two
sets, crossing one another obliquely. Both sets are inserted into the end of the teeth,
but whereas one set radiates from the base of the tongue (that is from directly below the
middle of the radula) the others run forward from the posterior end of the radula. By
acting together or on one side only these muscles can raise or depress the radula, alter
its curvature, and move it from side to side. Under the tongue are two semicircular bands
of muscle. The lower and larger is attached to the inferior surface of the jaws at one end
and to the hind part of the tongue at the other, so that it can pull the tongue downwards
and backwards. The other, which is smaller and lies within the curve of the lower
muscle, acts in the opposite way and is attached to the front part of the tongue, which it
can pull downwards and thus advance the middle part of the organ.