BRITISH NUDIBRANCHIATE MOLLUSCA. 35 



circulation, and respiration, be erroneous, the affinities of this tube with the Medusidse must 

 fall to the ground. In the Planarice, too, the ramifications of the digestive system are 

 apparently very different from those of the Eolidida. M. de Quatrefages lays much stress 

 upon the symmetrical and bilateral character possessed by the Bolides, which, he conceives, 

 approach them very closely to the Annelida. We do not see much force in this observation. 

 Most of the organs of the Mullusca possess normally a bilateral structure ; and it is principally 

 in those parts of the Gasteropods which are usually inclosed in a spiral shell that the 

 arrangement is departed from. In proportion as the shell becomes a concave or flattened 

 disc, or entirely disappears, the animal generally assumes a more symmetrical character. In 

 few families, indeed, is this more observable than in some of the ZJolidida, and in this respect 

 undoubtedly they may be considered to make a slight approach to the Annelida, as well as to 

 the other tribes whose bilateral structure is more perfect ; but this relationship cannot be 

 looked upon as of much importance ; nor can we perceive any real resemblance to the 

 segment of annulose animals in the regular branching of the gastro-hepatic vessels. This 

 system in the Zolidida, to a certain extent, resembles the prolongations of the digestive 

 organ in the Nymphons ; but here also we see no reason for supposing that there is any true 

 affinity between the two groups. 



The intimate relationship of the NudibrancHata with the other Gasteropods having been 

 pointed out, it remains to inquire what rank they ought to hold in this Molluscan group. 

 The great concentration of the ganglions in the nervous system would appear to indicate a 

 higher place in the scale than is borne out by their other characters. The olfactory organs 

 also reach their highest development in some of the genera of this order. But, on the other 

 hand, the eyes, though formed on the usual type, being constantly buried under the skin, 

 must be less effective instruments of sight than in those orders where they are raised on 

 pedicels, and assisted by the modification and transparency of the epidermal layer. The 

 simplification of the special breathing organs in some of the genera, taken in connexion with 

 their entire disappearance in the nearly-allied Pellibranchs, and the great simplicity of form in 

 the latter, undoubtedly imply a lower degree of organisation than in most of the Gasteropods, 

 though by no means to the extent that has been assumed in those views, we have so often had 

 occasion to combat in these pages. The hermaphrodism common to the whole of the order, 

 and the peculiarities in their embryological development already pointed out, lead to the same 

 conclusion. Upon the whole, therefore, it will be necessary to assign to the Nudibranchiate 

 Mollusca a place amongst the lowest members of the class Gasteropoda. 



Arrangement. — In reviewing the different arrangements of the Nudibranchiata, it will 

 not be necessary to go further back than the ' Regne Animal' of Cuvier, with whom the order 

 originated. In that work, the "Nudibranches " constitute the first order of the class Gastero- 

 poda in a descending series. The genera, which are not divided into families, stand thus : — 

 Boris, Polycera, Tritonia, Tkethjs, Scyllma, Glaucus, Uolidia, and Tergipes. Very little alteration 

 was made by Lamarck in his 'Animaux sans Vertebras.' He included these animals in a 

 single family, — "Zes Tritoniens" — reversing the order of the genera to suit his ascending 

 arrangement, but still placing them first, consequently lowest, among the Gasteropoda, 

 though in Cuvier's system they formed the highest group. Ferussac, adopting the orders as 

 well as the descending arrangement of Cuvier, divided the Nudibranchs into two sub-orders 

 and three families, as follows : — 



