Z 
106 NUDIBRANCHIATA.—DORIDID&. 
themselves to deep water, and are to be obtained 
only by dredging. 
GENUS DORIS. 
The body in this, the principal genus, is usu- 
ally of a semi-oval form sometimes very convex, 
‘but more generally depressed, and occasionally 
almost flat. The mantle is ample, projecting over 
the head, and on all sides; its surface is generally 
rough, with numerous warts, and its texture is stif- 
fened with calcareous spicula or crystals of lime im- 
bedded in its substance. The mouth is commonly 
without jaws, but is frequently furnished with a pre- 
hensile collar, armed with minute spines. ‘The tongue 
is covered with rows of teeth curved backwards. 
The breathing organs consist of flat plumes, 
sometimes small and 
simple, sometimes large 
and branching, either 
united at the base mto 
a flower-like expansion, 
or placed separately in 
a circle more or less 
perfect. In some species 
these plumes are capable 
of being withdrawn into 
a common cavity, the 
margin of which can be 
closed completely over 
them, but in others there 
is no cavity ; and when 
danger threatens, the 
BRANCHIAL PLUME OF DORIS. plumes are merely con- 
tracted, curled up, and bent down towards the 
centre of the circle. 
