In the development of a distinct breathing organ, it would seem as if progress had been made along two definite lines, each resulting in the exposure of a larger length of veins, _i.e._ of a larger amount of blood, to the simultaneous operation of fresh air or fresh water. Either (_a_) the skin itself may have developed, at more or less regular intervals, elevations, or folds, which gradually took the form of papillae, or else (_b_) an inward folding, or ‘invagination,’ of the skin, or such a modification of the mantle-fold as is described below (p. 172) may have taken place, resulting in the formation of a cavity more or less surrounded by walls, within which the breathing organs were ultimately developed. Sometimes a combination of both processes seems to have occurred, and after a papilliform organ has been produced, an extension or prolongation of the skin has taken place, in order to afford a protection to it. Respiration by means of a lung-cavity is certainly subsequent, in point of time, to respiration by means of branchiae. [Illustration: FIG. 59.--_Chiton squamosus_ L., Bermuda: =A=, anus; =Br=, branchiae; =M=, mouth.] [Illustration: FIG. 60.--_Fissurella virescens_ Sowb., Panama, showing position of the branchiae: =Br=, branchiae: =E=, =E=, eyes; =F=, foot; =M=, mantle; =T=, =T=, tentacles.] The branchiae seem to have been originally paired, and arranged symmetrically on opposite sides of the body. It is not easy to decide whether the multiple form of branchia which occurs in _Chiton_ (Fig. 59), or the simple form as in _Fissurella_ (Fig. 60), is the more primitive. Some authorities hold that the multiple branchia has gradually coalesced into the simple, others that the simple form has grown, by serial repetition, into the multiple. There appears to be no trace of any intermediate forms, and, as a matter of fact, the multiple branchia is found only in the _Amphineura_, while one or rarely two (never more) pairs of branchiae, occur, with various important modifications, in the vast majority of the Mollusca. _Amphineura._--In _Chiton_ the branchiae are external, forming a long row of short plumes, placed symmetrically along each side of the foot. The number of plumes, at the base of each of which lies an osphradial patch, varies from about 70 to as few as 6 or 7. When the plumes are few, they are confined to the posterior end, and thus approximate to the form and position of the branchiae in the other Amphineura. In _Chaetoderma_, the branchiae consist of two small feather-shaped bodies, placed symmetrically on either side of the anus, which opens into a sort of cloaca within which the branchiae are situated. In _Neomenia_ the branchiae are still further degraded, consisting of a single bunch of filaments lying within the cloaca, while in _Proneomenia_ there is no more than a few irregular folds on the cloaca-wall (Fig. 61).