2 A MONOGRAPH OF THE 



situated. But the characteristic peculiarity of these mollusks is the appendages that constitute 

 their breathing organs, placed upon the back, always symmetrically, in plumes, tufts, or 

 papillae, either forming a circle on the central line, or arranged in rows upon the sides. 



Unfortunately, the Nudibrancldata are as perishable as they are beautiful, it being 

 impossible to preserve them after death in their original forms and colours. From this 

 circumstance it is that they are so little known to the generality of persons, and that specimens 

 are seldom to be found in museums. The shapeless and colourless mass which most of them 

 present when preserved in spirits, is, indeed, a very poor representation of the living animal : 

 yet even in this state, imperfect as it is, they are useful for the examination of the zoologist, 

 as the characters may generally be observed by which the species are distinguished ; not to 

 mention the value of such specimens for the purpose of dissection. It is much to be regretted, 

 therefore, that these animals are not more frequently preserved, and that scientific travellers 

 of our own nation have so seldom paid attention to the foreign species, either by preserving 

 specimens for examination, or by making drawings of the living animals on the spot ; both of 

 which are necessary for a proper knowledge of the genera and species. 



None of the Nudibranchiate Mollusca appear to have been known to the ancients, and 

 even up to the time of Linnseus, they remained, with one or two rare exceptions, entirely 

 unnoticed. In the 12th edition of his ' Systema Naturae,' 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 Vermes, and referred to the genera Doris, Bcyllcea, and Tetliys. 

 That excellent observer, Otho Frederic Miiller, paid more attention to them : twelve species 

 are characterised in his ' Zoologise Danicse Prodromus/ 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 Fabricius has also excellent descriptions of two or three of these 

 mollusks in his ' Fauna Grcenlandica.' Other authors contributed a little to increase the 

 number of species, among whom Bomme, who described several in the Flushing Transactions, 

 is deserving of honorable mention. But these sparing contributions — few and far between — 

 were not sufficient to attract the general attention of naturalists to a group of animals difficult 

 of observation, and whose physiology and habits were as yet entirely unknown. It was 

 not until the appearance of the celebrated ' Memoires' of Cuvier, in the 'Annales da Museum,' 

 that much attention was drawn to this subject. These formed a new era in their history, and 

 the dissections there given furnished the groundwork for those more correct views of their 

 affinities which that distinguished naturalist carried out in the * Regne Animal/ where the 

 order Nudibrancldata 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 Linnean arrangement, 

 began now to be generally acknowledged. Their affinities were further illustrated in the 

 celebrated ' Histoire Naturelle des Animaux sans Vertebres,' 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. 



Let us now see what had been done during this time by the naturalists of our own 



