of the animal kingdom. They further express an opinion that the facts upon which is founded
the doctrine “ that the form of the body and the internal organisation are independent of each
other,” are not real, and that that hypothesis cannot be any longer maintained.
Prior to the appearance of these reports, a series of very excellent papers had been
published by M. Milne Edwards, taking an extended review of the circulatory system in the
Mollusca. He states his conviction, founded on extensive researches undertaken for the
purpose, that the venous system is incomplete throughout the whole of this large division of
the animal kingdom. In all instances he finds the true veins more or less imperfect, their
place being supplied by a series of lacunes, and the blood in most cases also flowing into the
abdominal cavity. The condition of the Nudibranchs he does not consider to be exceptional.
The difference of opinion amongst these distinguished naturalists with respect to the
circulatory system, has resolved itself into a very subtile anatomical fact. Both parties admit
the existence of large cavities into which the blood flows, but, on the one part, they are
considered as mere expansions of the vessels into blood-sinuses, and hence the vascular
system is uninterrupted; while on the other, they are held to be lacunes or gaps in the
continuity of the vessels, showing a yet imperfect state in the structure of these organs, and thus
forming an intermediate stage in the development of the vascular system, between its first
imperfect appearance in the lower animals, and the complete system of closed vessels, only to
be found, according to M. Milne Edwards, in the Vertebrata.
A very elaborate monograph of a species of Tergipes, found on the shores of the Black
Sea, was submitted to the Academy of St. Petersburg, by Professor Nordmann, of Odessa, in
1844, and appeared in the ‘Annales des Sciences Naturelles/ in 1846. M. Nordmann5s view of
the digestive and circulatory systems appear to agree pretty nearly with those of Messrs. Milne
Edwards and De Quatrefages. The liver, however, he had entirely misunderstood. He had
observed auditory capsules and otolithes, and that the ovate vesicles of the papillae occasionally
discharge tCa kind of mucus.” He gives a full' account'of the embryology, and has traced the
ulterior development of the young animal- further than had been done by other authors.
It is unnecessary here to analyse many excellent essays on the Nudibranchiata that
appeared nearly contemporaneously with or subsequent to those already mentioned, evincing
the interest that these little mollusks had excited among European naturalists. The papers of
Professor Allman ‘ On the Anatomy of Actaon, with remarks on the Phlebcnterata' and of Dr.
John Reid ‘ On the Development of the Nudibranchiate Mollusca/ in the ‘Annals of Natural
History/* and those of M. E. Blanchard, * Sur l’Organisation des Opisthobranches/ in the
‘Annales des Sciences Naturelles/t may be cited as affording many interesting details of
anatomy and development. So far as these authors touch upon the question of Phlebenterism,
their opinions are generally more or less opposed to those of M. de Quatrefages. In a paper
on the ‘ Anatomy of Doris ’ by Mr. Hancock and Dr. Embleton, read at the Edinburgh
Meeting of the British Association, in 1850, and since published in a much enlarged form in
the ' Philosophical Transactions of the Royal Society/ a portal heart and a sympathetic system
of nerves were shown to exist in the Nudibranchiata, being the first time that the existence of
these organs has been fully demonstrated in any of the invertebrated animals.
From this brief history of the study of these Mollusks it will be seen that many important
* Vol. 16, p. 145, and vol. 17, p. 377. + 3d ser., vol. 9, p. 172, and vol. 11, p. 74.
points in their anatomy and physiology have been elucidated by recent investigations. We
shall now proceed to give a more detailed account of the present state of our knowledge on
the subject. *
Anatomy. The Nudibranchiata exhibit a high state of organisation, not much inferior
to that of any of the Gasteropods ; and as, perhaps, in no other Molluscan group has the anatomy
been so perfectly investigated, we feel ourselves in a position to take a comprehensive
view of their structure, at least so far as the British forms are concerned. Our references to
foreign species must necessarily be very limited; as comparatively few of them have been dissected,
and even their external characters are, in too many instances, very imperfectly known.
Fortunately, however, the British list comprises types of all the larger groups and of a great
variety of the smaller ones,- so that we are enabled, from our indigenous species alone, to attain
a very accurate knowledge of the organisation of the whole order.
Alimentary System. All the Nudibranchs are provided with a powerful muscular buccal
apparatus, which has, in some instances, appended to it a gizzard, as in Lamellidoris, Gonio-
doris, Idalia, and others. The oral aperture is always guarded by fleshy lips, and the mouth
itself is furnished with a tongue, bearing a spiny prehensile membrane, and occasionally with
lateral corneous jaws. Tethys is the only exception, and in it there are neither jaws nor tongue;
neither is the buccal organ so muscular npr so distinctly defined as usual. The jaws are highly
developed in the Tritoniada and the Eolidida; in the former they are always present, in the
latter sometimes wanting, as is the case in Doto, Hermaa, Stiliger, and Aldena. A few of the
Polyccrina are also furnished with lateral jaws; but they are small, and do not appear to be
very efficient cutting instruments. JEgirus has an upper corneous jaw or tooth resembling
that of Uma x; and a minute rudimentary under jaw may be detected in some of the Borides.
The tongue is composed of a muscular apparatus bearing a stiffish membrane, furnished
with small teeth or spines. These are divided into two kinds, central and lateral, distinguished
by their position, and generally, when both are present, by a difference in form. The former
have been called dentes by Professor Loven, the latter undni; and the portions of the tongue
on which they are placed, are distinguished by that naturalist, under the names of rliachis and
pleura respectively. We have not thought it necessary to preserve these distinctive appellations.
In thq Boridida, the lateral spines are always developed, the central only occasionally.
Both kinds are present in the Tritoniada. The lingual membrane in the typical Borides, and
in the Tritoniada, is very broad, and is supplied with numerous spines; it is narrow in Lamellidoris
and Acanthodoris, there being very few spines in each transverse row,—in some of the
species as few as four. In such, the whole of the lingual spines do not amount to more than
112, while in Boris tuberculata there are no less than 6,000, and in Tritonia Hombergii,
upwards of 36,000. In the Polycerina, the lingual membrane is mostly narrow, and devoid of
central spines, and with one, two, or three, large, recurved spines on each side next the median
line. External to these, there are generally a few depressed plates or rudimentary spines.
JEgirus and Ceratosoma? are, however, exceptions, for in them the spiniferous membrane is broad,
bearing numerous spines, similar to those of Boris.
The lingual membrane in the typical Eolidida is very narrow, being furnished with a
longitudinal series of central plates or spines. In the sections of Eolis represented by E. tricolor
and E. rufibranchialis, however, there is on each side a single additional lateral spine,