16 BRITISH MARINE TESTACEOUS MOLLUSCA : 
this division, and which plants them im it for ever, after our 
method, is their mutual hermaphroditism. 
Some malacologists isolate these discordant groups from 
the systemic line, and term them Opisthobranchiata, from the 
heart being anterior to the branchiz, im opposition to the Proso- 
branchiata, which have the heart, in the greater part of the 
Gasteropoda, posterior to the respiratory organs; this arrange- 
ment causes Pleurobranchus, Aplysia, and Bulla to enter into 
line after the Cypreade,—a more unnatural position, we think, 
—at least it is an isolated one,—than that we have adopted ; 
and as there are other inconveniences in this plan, we have 
preferred, however incongruous and aberrant they may be 
with what follows, or precedes, or with themselves, to deposit 
them in the general line after the Patellide; and we are 
further induced to adopt this arrangement, as they are thereby 
at once brought within the limits of an important sexual 
scheme, which we think more conducive to a sound natural 
position than the posterior or anterior relation of the heart to 
the branchiz. Nature herself is here discordant, and will not 
allow us to symmetrize her works as the squares of a chess- 
board. This is a vain idea, and however often repeated, will 
as signally fail as the hopeless tasks of the Belides. 
The Nudibranchiata, which have neither external nor in- 
ternal testaceous appendages, do not come within the scope of 
our work,—they are only mentioned as a numerous molluscan 
tribe; their position as hermaphrodites is in the Third and 
Fourth Divisions. 
The Tunicata, comprising the Ascidiade, although they 
have some points of alliance with the Mollusca, we do not 
consider entitled to range with that class. 
Fifth Division. 
GASTEROPODA PECTINIBRANCHIATA. 
Bisexual. 
The genera Cyclostoma, Acme, Paludina, Neritina, the 
Littorinide, Tornatellide, Heterophrosynide, Pyramidellide, 
Peloride, Alatide, Muricide, and Cypreade are probably all 
