In _Patella_ the breathing arrangements are very remarkable. In spite of their apparent external similarity, this genus possesses no such symmetrically paired plume-shaped branchiae as _Fissurella_, but we notice a circlet of gill-lamellae, which extends completely round the edge of the mantle. It has been shown by various authorities that these lamellae are in no sense morphologically related to the paired branchiae in other Mollusca, but only correspond to them functionally. The typical paired branchiae, as has been shown by Spengel, exist in _Patella_ in a most rudimentary form, being reduced to a pair of minute yellow bodies on the right and left sides of the back of the ‘neck.’ A precisely similar abortion of the true branchiae, and special development of a new organ to perform their work, is shown in _Phyllidia_ and _Pleurophyllidia_ (see below under Opisthobranchiata). This circlet of functional gills in _Patella_ has therefore little systematic value, being only developed in an unusual position, like the eyes on the mantle in certain _Pelecypoda_, to supply the place of the true organs which have fallen into disuse. Accordingly Cuvier’s class of _Cyclobranchiata_, which included _Patella_ and _Chiton_, has no value, and has indeed long been discarded. In _Chiton_ the gills never extend completely round the animal, but are always more or less interrupted at the head and anus. They are the true gills, the plumes being serially repeated in the same way as the shell plates. [Illustration: FIG. 63.--_Patella vulgata_ L., seen from the ventral side: _f_, foot; _g.l_, circlet of gill lamellae; _m.e_, edge of the mantle; _mu_, attachment muscle; _sl_, slits in the same; _sh_, shell; _v_, vessel carrying aerated blood to the heart; _v´_, vessel carrying blood from the heart; _ve_, small accessory vessels.] [Illustration: FIG. 64.--_Patella vulgata_ L., seen from the dorsal side after the removal of the shell and the black pigment covering the integument; the anterior portion of the mantle is cut away or turned back: _a_, anus; _br_, _br_, remains of the true branchiae (ctenidia); _i_, intestine; _k_, _k´_, kidneys; _k.ap_, their apertures on each side of the anus; _l_, liver; _m_, _m_, mantle; _mu_, attachment muscles, severed in removal of shell; _t_, _t_, tentacles.]