16 BEITISH MARINE TESTACEOUS MOLLUSCA : 



this division, and which plants them in 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 branchiae, in 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 Bull a to enter into 

 line after the Cypr&ada, 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 Patellida; 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 branchiae. 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 Ascidiadce, 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 

 Littorinidce, Tornatellida, Heterophrosynidte, Pyramidellidce, 

 Pelorida, Alatidce, Muricida, and Cypr&adce are probably all 



