and warningly coloured in different attitudes. It is one of the largest and most active of
tropical Nudibranchs, having a broad, bright red mantle-margin, and in the centre of the
back an area coloured by various shades of yellow. When it displays the margin it is
exceedingly conspicuous. When it folds over the flexible red margin and exposes only
the yellow back, it is hardly visible on a sandy bottom.
The distastefulness of some Nudibranchs perhaps depends on their power of secreting
mucus. Many species can pour forth this secretion in abundance sufficient not merely to
render them unpalatable but to entangle and hamper a small animal with appendages,
such as a crustacean or worm. Several forms, such as Doto, Gahna glaucoides, Proctonotus
mucroniferus and Elysia viridis have special glands, which appear to discharge an
offensive secretion, and this is probably the function of the glands found on the branchiae
of many Dorids.1 They disgust inquisitive enemies who may be disposed to nibble these
delicate organs. But by far the most effective and remarkable arms of defence are the
nematocysts found in many Cladohepatica,1 2 and the proof of their origin, due to Mr.
Grosvenor3 * and Professor Cu&iot, is the most important contribution to the biology of the
group made since the time of Alder and Hancock. In most -Solids there is present in
the tips of the cerata a sac of endodermic origin, which communicates directly with the
exterior, and also, by means of a ciliated canal, with the diverticula of the liver which
form the core of the cerata. This cnidosac, as it is called, usually contains nematocysts
or thread-cells. These are minute capsules containing a fluid in which is coiled a long
thread. Under a suitable stimulus they pass out of the cnidosac into the water, evert
the thread, and are capable of stinging any delicate animal. I t had long been known that
the nematocysts of iEolids are similar to those of various Ccelenterata (.Actiniae, Tubularise,
etc.) on which they feed, and as early as 1858 Strethill Wright maintained that they passed
into the molluscs from their prey, but for a long while the explanation was thought too
extraordinary to be credible, and the nematocysts of the JEolid were supposed to be
developed within its own cnidosacs. But thanks to the researches mentioned it may be considered
proved that they are not produced from the tissues of the ^Eolid, but are acquired
by it from Ccelenterata in the process of feeding, and pass through the stomach and
hepatic diverticula into the cnidosacs without being digested. That such a passage is
possible is shown by the fact that other indigestible matter, such as the radulaa of small
molluscs, has been observed in the cerata of ^Eolids. Such strange phenomena require
the confirmation of careful observation and experiment, but the evidence is clear.
(1) Nematocysts are of many distinct kinds. Several kinds may occur in one JEolid, and
individuals of the same species may have different kinds of nematocysts. (2) I t is
shown by experiment that the cnidosacs always contain nematocysts similar to those of
the Coelenterate on which the iEolid has been recently feeding, and that the nematocysts
found in the hepatic diverticula and faeces are similar. (3) The nematocysts of an ^Eolid
can be changed by changing its food, and those ^Eolids which habitually feed on animals
1 See Hecht 1, pp. 596—604.
. 2 In most HSolids, and in nearly all Pleurophyllidiidte.
3 See Grosvenor, “ On the Nematocysts of S o lid s/5 Proc. Roy. Soc., 1903, pp. 462 486.
Glaser, “ Nematocysts of Nudib. Moll.,” J. Hopkins Univ. Circ., xxii, p. 22, 1903. Cnenot,
“ L’origine des nematocystes des Eolidiens,” Arch, de Zool. exper., 1907, pp. 73—102.
which have no nematocysts have none themselves.1 But the phenomenon, though so
clearly attested, remains extraordinary and, indeed, unique. The nearest parallel is the
behaviour of the Cephalopod Tremoctopus microstoma, which is said to carry on its arms
fragments of a stinging medusa. But there is no real analogy for the functions here
assumed by the alimentary canal, which accepts substances not in order to digest or take
them into the system but in order to eject them by special orifices as a means of defence.
Very unusual too is the plan of the alimentary system, which communicates with the
exterior not merely by the mouth and anus but by numerous orifices, in some cases
hundreds.2 It does not appear that an JEolid uses its nematocysts for paralyzing its prey
or that it discharges them against advancing enemies. When attacked it contracts its
body, at the same time erecting and sometimes lengthening its cerata, so that the assailant
can hardly help touching them. When bitten or squeezed they" pour forth the stinging
nematocysts; the assailant probably withdraws and the -ZEolid probably escapes with the
loss of some of its cerata, which seems to be no inconvenience. Most (though not all)
fish reject JEolids, hut it is not certain whether the mucus or the stinging is the more distasteful
feature. I t seems that those ^Eolids which have no nematocysts, such as Galma
and Fiona, have specially abundant mucus or offensive glands.3
It is clearly to the advantage of ^Eolids and other animals of a similar structure that
the cerata should not be vital parts and should be detachable without serious injury, for
unless the animal is swallowed whole these are the parts which an adversary is bound to
seize at the first attack. Accordingly we find that in most (probably all) ^Solids the
cerata can be torn off by slight violence, and in some detach themselves spontaneously
when the animal is annoyed. Among British species this autotomy is common in Tergipes
despectus, Galvina exigua, Doto and Antiopella. The; last named, which in its normal
condition has numerous large cerata, is sometimes found covered with minute ones and
presenting a totally different appearance. These are young cerata in course of regeneration
replacing those which have been thrown off. In Proctonotus the cerata are not only
easily detachable but have a special apparatus at the top which enables them to adhere to
extraneous objects. They are also richly provided with glands, and thus when an enemy
seizes them they adhere and annoy him with their secretion while the body of the
Proctonotus crawls away. Similar autotomy is not recorded of any British Dorid, but in
the tropics Discodoris fragilis and others are capable of casting off the whole mantle-
margin and remain in apparent good health.
The Nudibranchiata, like most marine animals, are infested by both external and
internal parasites. Though these unbidden guests attain a relatively considerable size,
they do not appear to injure their hosts as a rule, for they attain maturity in full-
1 Many other confirmatory details and experiments are given by Cuenot and Grosvenor (l. c.). It
is a pity that all the observations were made on AEolids and did not include the very differently
shaped Pleurophyllidiidse, which also have nematocysts.
3 A somewhat similar arrangement occurs in some Turbellaria such as Yu/ngia and Gycloporus.
In these animals the digestive cavity exhibits numerous branched diverticula which extend to the
integuments and communicate with the exterior by terminal openings. In Gycloporus these openings
are said to occur close together around the entire margin of the body.
8 So, too, Eermsea and Proctonotus.