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GASTEROPODES. 
Tur Doris.—Plate XX XVIII. 
_ 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 7ritonia, 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. 
Il. The special form of some of its principal parts, such as the head 
or the sole. 
II. 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 branchiz. 
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 
