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.
Fig. 59.—Chiton squamosus L., Bermuda: A, anus; Br, branchiae; M, mouth.
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]).
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).