[Illustration: FIG. 61.--Terminal portions of the Amphineura, illustrating the gradual degradation of the branchiae, and their grouping round the anus in that class. =A=, _Chiton_ (_Hemiarthrum_) _setulosus_ Carp., Torres Str.; =B=, _Chiton_ (_Leptochiton_) _benthus_ Hadd., Torres Str.; =C=, _Chaetoderma_; =D=, _Neomenia_; _a_, anus; _br_, _br_, branchiae; _k_, _k_, kidneys; _p_, pericardium. (=A= and =B= after Haddon, =C= and =D= after Hubrecht.)] In the _Prosobranchiata_, symmetrically paired branchiae occur only in the Fissurellidae, Haliotidae, and Pleurotomariidae, in the former of which two perfectly equal branchiae are situated on either side of the back of the neck. These three families taken together form the group known as _Zygobranchiata_.[268] In all other families the asymmetry of the body has probably caused one of the branchiae, the right (originally left), to become aborted, and consequently there is only one branchia, the left, in the vast majority of marine Prosobranchiata, which have been accordingly grouped as _Azygobranchiata_. Even in _Haliotis_ the right branchia is rather smaller than the left, while the great size of the attachment muscle causes the whole branchial cavity to become pushed over towards the left side. In those forms which in other respects most nearly approach the Zygobranchiata, namely, the Trochidae, Neritidae, and Turbinidae, the branchia has two rows of filaments, one on each side of the long axis, while in all other Prosobranchiata there is but one row (see Fig. 79, p. 169). [Illustration: FIG. 62.--_Bullia laevissima_ Gmel., showing branchial siphon =S=; =F=, =F=, =F=, foot; =OP=, operculum; =P=, penis; =Pr=, proboscis; =T=, =T=, tentacles. (After Quoy and Gaimard.)]