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ORDER—PROSOBRANCHIATA. M. Epwarps. 
PECTINIBRANCHIATA, Cuvier. 
TUBULIBRANCHIATA, 
” 
ScUTIBRANCHIATA, oS 
CYCLOBRANCHIATA, 
” 
PARACEPHALOPHORA DIoIcA, De Blainville. 
To the free-air-breathing gasteropods succeed those which breathe, by means of 
gills, the air diffused through the water in which they live. In them the head is more 
or less fully developed, and the mouth is furnished with a riband-shaped tongue, 
armed with numerous series of teeth, which present great varieties of form and 
arrangement. In some cases the animals are hermaphrodite, the sexes being united 
in the same individual, but in by far the larger proportion the sexes are distinct ; with 
very few exceptions, they are all oviparous. In the larva state they are always 
furnished with spiral shells, which, in some cases, as the animals approach maturity, 
become rudimentary or altogether disappear; but more generally the shells become 
largely developed, so as to contain the whole animals within them. The respiratory 
organs exhibit many differences in structure and position,and these varied conditions were 
adopted by Cuvier as ordinal distinctions in the systematic arrangement proposed by him. 
De Blaimville, on the other hand, availed himself of the modifications in the repro- 
ductive apparatus, and divided his second class “‘ paracephalophora,” into the sub-classes 
dioica, in which the male and female sexual organs are in different individuals, and 
monoica, in which the two sexes are united in the same individual. To these he added 
a third division, ermaphrodita, in which he described the generative apparatus as 
female only, a modification the existence of which subsequent investigation has 
disproved. It appears, however, by the observations of Milne Edwards, that the 
water-breathing gasteropods form two natural and well-defined divisions, which that 
eminent naturalist has called respectively, opisthobranchiata and prosobranchiata, from 
the position of the gills in relation to the heart.* In the first of these divisions, which 
corresponds with the zudibranchiata, testibranchiata, and inferobranchiata of Cuvier, and 
with the monoica and hermaphrodita of De Blainville, the respiration is effected by 
* Etym. omo(e (in the after part, behind), and zpowsov vel zp@cov (advanced, pushed forward), 
respectively prefixed to Bpayyia (the gills). 
