272 



NUDIBRANCHES. 



Fig. 46. 



Cuviera, copied from Rang. 



Fig. 47. 



of two small equal and symmetrical processes, exsertile be- 

 yond the shell when the animal is in motion, but at other 



times retracted, and fixed upon a 

 common stalk by a point a little 

 removed from one of the ends.* 



The Gasteropoda which arrange 

 themselves under this division form 

 a very natural and interesting 

 order, appropriately named by Cu- 

 vier the Nudibranches. These are 

 naked snail-like mollusca which 

 live only in the sea; and they would 

 scarcely attract our notice amid 

 the myriads of curious creatures 

 that are around them, were it not 

 for the ornament and singularity of 

 their branchial appendages. Their position is always on 

 some part of the back, either ranged in one or more series 



along its margins, as in Glaucus, 

 Eolis, and the Tritoniadse ; or 

 clustered on a point of the medial 

 line near the hinder extremity, 

 as in the Doris and its allies, which 

 have the power of concealing 

 them within the body when dan- 

 ger threatens from without. In 

 shape they vary more than in 

 position : they are simple fila- 

 ments in Eolis (Fig. 47) ; in 

 Glaucus they are fan-shaped fins ; 

 in Melibcea clubbed processes, 

 covered with little hispid tuber- 

 cles, or, as in Scyllea, with little 

 tufted bouquets of very delicate 

 filaments ; and in the Tritoniadae 

 and Doris they assume a more 

 or less perfectly plumose or arborescent appearance. f 



* Rang's Manuel de 1'Histoire Naturelle des Mollusques, p. 38. 



t In reference to the papillary appendages of Eolis, and of the genera 

 which it represents, Mr. Couthouy conjectures that they '-'are not the real 

 respiratory organs, because he has seen that the animal will voluntarily 

 throw them off, from slight causes, or that it may be forcibly deprived of 

 them without material injury ; which, he justly remarks, would not be 

 likely to be the case, were they organs of so much importance as the bran- 

 chiae. He is disposed to regard them as merely subsidiary to the function 

 of respiration." GOULD'S Invertebrata of Massachussetts, p. 6. Quatre- 

 fages has endeavoured to prove the organs in question to be connected with 



