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ORDER— PROSOBRJNCHIATA. M. Edwards. 



Pectinibranchiata, Cuvier. 

 tubulibranchiata, „ 

 scutibranchiata, „ 

 Cyclobranciiiata, ,, 

 Paracephalophora diotca, 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 andposition,and these varied conditions were 

 adopted by Cuvier as ordinal distinctions in the systematic arrangement proposed by him. 

 De Blainville, on the other hand, availed himself of the modifications in the repro- 

 ductive apparatus, and divided his second class " paracephalojohora" into the sub-classes 

 diotca, 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, hermaphrodita, 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 nudibranchiata, testibranchiata, and inferobranchiata of Cuvier, and 

 with the monoica and hermaphrodita of De Blainville, the respiration is effected by 



* Etym. oTriade (in the after part, behind), and irfioiuoov vel npwoov (advanced, pushed forward), 

 respectively prefixed to /3payx ta (the gills). 



