34 A MONOGRAPH OF THE 



situated in advance of the frontal margin of the cloak ; the anal opening is lateral, and the 

 skin is destitute of spicula. But this genus makes a near approach to the Nudibranchs in the 

 arrangement of the digestive apparatus, having a gastro- hepatic system, though the modifica- 

 tion of the parts is somewhat peculiar. In taking an extended view of the order, these two 

 genera ought perhaps to be included in it.* 



There is yet another order with which the Nudibranchs might be supposed to come into 

 contact, through what, at first sight, appears to be an inosculating genus — the Pulmonata. 

 Among the naked slugs, the marine genus Onchidium {Peronia, Blainv.) shows some approach 

 to the Nudibranchiate tribes ; but this is more apparent than real. In form it certainly very 

 much resembles Boris, and, though the specialised breathing organ is a pulmonary cavity 

 placed posteriorly beneath the cloak, yet there cannot be a doubt that, as in the Nudibranchs, 

 the skin partakes largely in this function. This we infer, both from its structure and from 

 the observed fact that the Onchidium Celticum has the power of remaining for a long period 

 under water, without the necessity of coming to the surface to breathe. In some of the 

 foreign species of this genus, the tubercles of the skin are prolonged, in the posterior part of 

 the cloak, into branched processes, very similar to the gills of Tritonia and Dendronotus. 

 This is remarkably the case in Onchidium punctatum [Peronia Alderi, Gray). There can be no 

 doubt, however, that Onchidium is organised on the type of the Pulmonata, whether we look 

 to the nervous, the digestive, or the reproductive system, as well as to that of respiration. 



In their mode of development the Nudibranchs resemble very closely Aplysia, Bulla, and 

 most of those genera of Tectibranchiata and Inferobranchiata in which the process has been 

 observed. The spawn is likewise very similar. This is generally in the form of a gelatinous 

 riband, assuming more or less of a spiral direction. The larva? of Aplysia and Bulla are 

 scarcely to be distinguished from those of Tritonia and Boris. These tribes differ from the 

 Pectenibranchiata and the other orders in which the young come forth in a form resembling 

 the mature animal, principally in the circumstance that the embryo issues from the egg in an 

 earlier stage of development, and that the metamorphosis which it undergoes in all cases, takes 

 place within the egg in the one instance, and after its extrusion from it in the other. The 

 latter circumstance appears to imply a lower stage in the zoological scale, as it is found to be 

 the rule in the lower classes of animals, though a very rare exception in the higher ones. 



Besides the more obvious affinities here mentioned, M. de Quatrefages has endeavoured 

 to show that a marked relationship exists between these Mollusks and most of the other classes 

 of Invertebrate Animals ; that, in fact, the "Phleb enter ala," having, so to speak, thrown off 

 many of the characters proper to the class to which they belong, have at the same time 

 assumed others, typical of other great divisions of the animal kingdom. Thus, he traces in 

 these Mullusks characters that ally them to the Medusida, the Annelidte, the Crustacea, and 

 even to the Infusoria. Believing as we do, that several of the characters given by M. de 

 Quatrefages to his a Phleb enter aid" are founded upon erroneous observations, we cannot of 

 course agree in the conclusions drawn from them. If, for instance, as we believe, the idea 

 that the branched appendages of the stomach perform the three functions of digestion, 



* Those authors who include the genera above mentioned amongst the Nudibranchiata do away 

 with the order Inferobranchiata, and unite Pleurobranchus and the remaining genera of that order with 

 the Tectibranchiata into an order, distinguished by having the gills on one side only, which they name 

 Pleurobranchiata. 



