r. 



NUDIBRANCHIATA. 



NTJDIBUANCniATA. 



upper pair of which arc placed on the cloak when it it present, and 

 behind them the eyes are situated. But the characteristic peculiarity 

 of these molluscs is the appendages that constitute their breathing 

 organs, placed upon the back, always symmetrically, in plumes, tuft*, 

 or papilla), either fanning a circle on the central line, or arranged in 

 rows upon the sides. 



None of the Nudibranchiate MvUiuca, appear to have been known 

 to the ancients, and even up to the time of Linnaeus they remained, 

 with one or two rare exceptions, entirely unnoticed. In the twelfth 

 edition of his ' Systema Natura) ' only seven species were described, 

 scarcely any of which had come under his own observation in a living 

 state. They were placed by him in the class Vermei, and reforred to 

 the genera Dora, Scylltea, and Tttltyi. Otho Frederic Miiller paid 

 more attention to them; twelve species are characterised in his 

 ' Zoologiae iJanicas Prodromiw,' most of which were afterwards figured 

 with fuller descriptions in the ' Zoologia Danica.' The number of 

 species introduced into the latter work is fourteen. Otho Fabric! u 

 has also excellent descriptions of two or three of these molluscs in his 

 ' Fauna Gramlandica.' Other authors contributed a little to increase 

 the number of species ; but it was not until the appearance of the 

 celebrated ' Mdmoires ' of Cuvier, in the ' Annales du Museum,' that 

 much attention was drawn to this subject. These formed a new era 

 in their history, and the dissections there given furnished the ground- 

 work for those more correct views of their affinities which that dis- 

 tinguished naturalist carried out in the ' Regne Animal,' where the 

 order Nudibranchiata was first instituted for their reception. Even at 

 that time however very few species were known, and it is to be regretted 

 that Cuvier was obliged to have recourse to specimens preserved in 

 spirits for his descriptions. So far as their anatomy was concerned 

 this disadvantage was not greatly felt, but the figures and descriptions 

 of their external forms were in consequence very imperfect The 

 position of this group in relation to the testaceous tribes, from which 

 they had been kept apart in the Linnauui arrangement, began now to 

 be generally acknowledged. Their affinities were further illustrated 

 celebrated ' Histoire Nature-lie des Animaux sans Vertobres ' of 

 Lamarck, and in the ' Manuel de Malacologie ' of Blainville ; each of 

 which contributed something to the knowledge of their physiology and 

 relations, but not much to the number of species. 



Although little had been done up to this time by British naturalists 

 in augmenting the species of this beautiful family, they have been 

 since the subjects of most accurate and fruitful research; and the 

 Monograph now publishing by the Ray Society on the ' British Nudi- 

 branchiate Mollusca', may be regarded as one of the most remarkable 

 contributions made to the literature of natural history during the 

 present century. This beautiful work contains the description of one 

 hundred species ; of these sixty have been added by Messrs. Alder and 

 Hancock, and their forms and anatomy illustrated by a series of coloured 

 plates from original drawings of the authors. 



We need not enumerate the additions that have been made to this 

 department of the European Fauna during the last half century; suf- 

 fice it to say that of late years much attention has been paid to the 

 NudAranchiata by continental naturalist), and several new species 

 have been described. Among the authors treating of the subject may 

 be mentioned Kisao, D'Orbigny, Cantrnine, Quatrefages, Bars, Lovdn, 

 Delle Chiaje-, I'hilippi, and Verany. No account however of the Nudi- 

 branchiate MoUutca of any one country has yet been published suffi- 

 ciently complete to form the basis of a comparison with our own. The 

 best are those of Lovt-n, ('Index Molluscorum Scandinavia'), who 

 gives thirty-seven species as members of the Scandinavian Fauna ; and 

 of \Vraiiy, whose Catalogue of the Mollusca inhabiting the Gulf of 

 Oenoa (' Catalogo degli Animali Invertebrati,' &c.), includes forty-eight 

 species of Nu.dihranchia.ta. The Sicilian species described by l'hili|>pj 

 are twenty-six (' Enumeratio Molluscorum Sicilies '). 



But it is not in a numerical point of view alone that our knowledge 

 of HIM interesting tribe of animals has increased; their anatomy and 

 pbyiiiology, their habits and alliances, have lately been studied with 

 care and attention, and many curious facts concerning them have been 

 ascertained. In 1841, the celebrated Norwegian naturalist, M. Sars, 

 announced the discovery that these little creatures undergo a meta- 

 morphosis, having on their extrusion from the egg a very different 

 form and character from those which they are afterwards destined to 

 assume. In this first stage of their existence they have the appear- 

 ance of small animalcules, swimming freely through the water by 

 means of two ciliated lobes, and have their body covered by a nautiloid 

 shell furnished with an operculum. Up to that time nothing 

 approaching to a distinct metamorphosis had been known to exist in 

 any of the true lUoUiuca ; the announcement therefore did not fail to 

 excite a considerable degree of interest. The investigation of this 

 n fact was pursued and extended by M. Lovc-n and other natu- 

 ralists, the result of which showed that this peculiar mode of develop- 

 ment was not confined to the Nudibrancbs alone, but was common to 

 many of the allied families ; the metamorphosis however is most 

 striking in those genera which, like the former, do not bear a shell in 

 tbt-ir adult state. 



Professor Milne-Edwards was the first to describe ('Annales des 



Science.* Natiirellen,' 2nd Series, vol. xviii., p. 330) a curious con- 



: the digestive organs in the family of the Eoltdidtt, the 



Unification and uses of which have since been the subject of 



much controversy. Having observed in a small Calliopcea, found at 

 Nice, a system of branched canals connected with the stomach, and 

 extending to the papillae and other parts of the external surface, he 

 thought he saw in this arrangement a blending of the functions of 

 digeatiori with those of the vascular system, which he in consequence 

 called gastro-vascular. This apparatus he compares to the system of 

 vessels radiating from the stomach of the Mcdurida on the one hand, 

 and to the caeca connected with the digestive organs of the NymphouB 

 among the Cnutacea on the other. 



During the same year (1842), M. Delle Chiajehad published a figure 

 of hU Eulit crittata (Antiopa critlala, A. and H.), in which a similar 

 apparatus of branching vessels connected with the stomach is repre- 

 sented, but without any letter-press description. This view was 

 adopted by M. Quatrefages, who founded on it his theory of the degra- 

 dation of certain forms of Afolltuca, which he placed in an order by 

 themselves, and which he called 1'hlebenterata. The views of M. Quatre- 

 fages, although at first favourably regarded by the French Academy, have 

 since met with much opposition ; and in a report made by a Commission 

 to the Biological Society in Paris, which was drawn up by Dr. Charles 

 Robin, it is stated that the Commission regards the idea of Phleben- 

 terism as untenable. They consider the ramifications of the digestive 

 system, which M. Quatrefages regarded as a gastro-vaitcular system, to 

 be true biliary ducts in connection with a divided liver, and that they 

 do not fulfil any other function than the usual one of that organ. 

 They moreover consider that the circulatory system in these animals 

 (Kolit, Actteon, &c.) is complete, the so-called lacunes being similar to 

 the blood-sinuses known to exist in particular cases throughout all 

 department* of the animal kingdom. They further express an opinion 

 that the facts upon which is founded the doctrine, enunciated by 

 M. Qoatrefages, " that the form of the body and the internal organi- 

 sation 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 Motlutea. He states his con- 

 viction, 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 seriex 

 of lacunes, and the blood in most cases also flowing into the abdomi- 

 nal cavity. The condition of the Nudibranohs he does not consider to 

 be exceptional. 



The difference of opinion amongst these distinguished naturalists 

 with respect to the circulatory system, lias resolved itself into a very 

 subtle 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 form- 

 ing 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 Vertelrata. 



The NwlibranchiiUa exhibit a high state of organisation, not mucli 

 inferior to that of any of the Gatteroptxla. They are all provided 

 with a powerful muscular buccal apparatus, which has, in some 

 instances, appended to it a gizzard, as in Lamcllidorit, (Joniodorw, 

 Jdatia, 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. 



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 Lovi'n, the Utter Uncini ; 

 and the portions of the tongue on which they arc placed, are 

 distinguished by that naturalist, under the names of Rhachis and 

 Pleura) respectively. The lingual membrane in the typical Harida, 

 and in the Tritoniadce, is very broad, and is supplied with numerous 

 spines; it is narrow in Lamellidorii and A canthodorii, 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 Dorit tubtmdata there are no less than 

 6000, and in Trilonia Uombergii upwards of 36,000. 



The oesophagus, stomach, and intestines are well marked ; the former 

 is generally short and passes from the upper surface of the buccal 



In;. | 



The stomach varies considerably in size and form; and in the 

 Uoridida in frequently buried in the liver. In them, too, there is 

 occasionally, besides a buccal gizzard, an anterior stomach or crop, 

 formed by a dilatation of the (esophagus. 



Thii intestine is always short. 



The liver presents two great tpyes of form. In the JJoridida and 

 Tritoniada it is entire (excepting in Scyllira, where it is broken up 

 into six or seven globular masses), occupying its normal abdominal 

 position ; in the Kululidtt it is more or less diffused. In those genera 

 with an entire liver it is very bulky, pouring the hepatic fluid into the 



