162 MOLLUSCA 
would certainly be an unexpected occurrence, to find a mar- 
ine gasteropodous mollusca obliged to come to the surface, 
at intervals, to respire. It will probably be found that it is 
truly branchiferous. 
Crass I].—BRANCHIFERA. 
The molluscous animals of this class are more numerous 
than those of the preceding. They chiefly inhabit the wa- 
ters of the ocean, a few genera only being met with in fresh 
water lakes and rivers. The branchiz which constitute their 
aérating organs, exhibit numerous varieties of form, posi- 
tion, and protection, and furnish valuable characters for their 
methodical distribution. 
OrvpER ].— BRANCHILE EXTERNAL. 
The branchiz are pedunculated, and more or less plu- 
mose. ‘They are moveable at the will of the animal, and, 
m general, are capable of great alteration of form. 
Genvs Doris.—Oral tentacula two; vent without scales. 
The cloak is covered with retractile papillz, and separated 
from the foot by a distinct duplicature. Towards its ante- 
rior margin are placed the two superior tentacula. These 
are retractile, surrounded at the base with a short sheath, 
and supported on a slender stem, having an enlarged com- 
pound plicated summit. The neck is short, and above the 
mouth there is a small projecting membrane, connected at 
each side with the oral tentacula, which are in general mi- 
nute, and of difficult detection. The mouth is in the form . 
of a short trunk, leading to fleshy lips, within which the 
tongue is placed. This last organ is covered with minute 
