270 GASTEROPODES. 



THE DORIS. Plate XXXVIII. 



The comparative simplicity of the preceding subject, whose nature 

 corresponds so intimately with that of the animals of this section, forms 

 a very suitable introduction to it. 



The great section established as the Doris by earlier naturalists, has 

 been partitioned by later observers into several distinct groups. But 

 there is no race of animals more difficult to subdivide satisfactorily, to 

 arrange, and to connect, than the Doris. Perhaps we have yet too few 

 intermediate links to unite the chain. Whence it is very questionable 

 whether the present divisions will retain their permanence. 



Without attempting any refined classification, however, a few gene- 

 ral observations on some of the animals comprehended within the sec- 

 tions Tritonia, Eolis, and Doris, as proposed by authors, may prove use- 

 ful to those devoting themselves to the subject. I have constantly found 

 varied novelties result from the enquiries of different naturalists, though 

 relating to the same subject. 



The basis of arrangement may rest on many different principles. 

 Such as, 



I. The general configuration of the animal. 



II. The special form of some of its principal parts, such as the head 

 or the sole. 



III. The number and formation of the tentacular organs. 



IV. The presence or absence of cloak or mantle, a total or partial 

 covering of the upper surface. 



V. The position and form of the branchiae. 



VI. The peculiar form and arrangement of the spawn. 



Of the features common to the whole tribe, are a number of external 

 organs : an extensive sole, whereon the individual crawls ; and horns in 

 two pair, one projecting from the head, to be denominated tentacula, 

 generally of simple structure ; a pair called cornicula, projecting above 



