In the land Prosobranchiata (Cyclostomatidae, Cyclophoridae, Aciculidae, Helicinidae) which, having exchanged a marine for an aerial life, breathe air instead of water, the branchia has completely disappeared, and breathing is conducted, as in the Pulmonata, by a lung-cavity. In certain genera of land operculates, e.g. Pupina, Cataulus, Pterocyclus, a slight fissure or tube in the last whorl (see Fig. [180], p. 266) serves to introduce air into the shell, which is perhaps otherwise closed to air by the operculum. In Aulopoma, which has no tube, the operculum admits free circulation of air. In certain other Cyclostomatidae the apex is truncated, and air can enter there. De Folin closed with wax the aperture of Cycl. elegans, and found that on placing it in a pneumatic machine, the shell gave off air through its whole surface. On the other hand, Cylindrella and Stenogyra decollata, on being submitted to the same test, showed that the truncated part alone was permeable by air.
Fischer and Bouvier have made some interesting observations on the breathing of a species of Ampullaria (insularum Orb.). The species has, in common with all Ampullaria, two siphons, but while the right siphon is but slightly developed, the left is very long, almost twice as long as the shell (see Fig. [65]). The animal, when under the water, lengthens its siphon, brings the orifice to the surface, and by alternately raising and depressing its head produces in the pulmonary sac movements of ex- and inspiration; these are repeated about ten or fifteen times at regular intervals of from six to eight seconds, a method of respiration strongly resembling that of the Cetacea. At the same time, branchial respiration takes place. If powdered carmine is added to water, the particles are seen to enter the branchial cavity by the siphon and pass out by the short right siphon. Sometimes the animal remains under water for hours without rising to the surface to inspire air. In Valvata (Fig. [66]) the branchia is very large, and projects like a leaf or fan above the shell on the left side; on the corresponding position on the right side is a long filiform appendage, which some have regarded as representing the other branchia.
Fig. 65.—Ampullaria insularum Orb.: A, breathing water; B, breathing air; Si, siphon; T, upper; t, lower tentacles; X, pallial expansion, performing the part of excurrent siphon. (After Fischer and Bouvier, x ⅓.)
Opisthobranchiata.—A true branchia occurs only in the Tectibranchiata and the Ascoglossa. It lies on the right side, and is usually more or less external, being partly covered sometimes by the shell (as in Umbrella, Fig. [5]), sometimes by a fold of the mantle. In the Pteropoda (which are probably derived from the Tectibranchiata), all the Thecosomata, with the exception of Cavolinia, have no specialised branchia, but probably respire through portions or the whole of the integument. In the Gymnosomata an accessory branchia has in many cases been developed at the posterior end of the body. Pneumodermon alone has both lateral and posterior branchiae well developed, Clione and Halopsyche are destitute of either, while the four remaining families have one branchia, sometimes lateral, sometimes posterior.[269]
Fig. 66.—Valvata piscinalis Müll.: br, branchia; fi, filament; f.l, foot lobes. (After Boutan.)
Fig. 67.—Doris (Archidoris) tuberculata L., Britain: a, anus; br, branchiae, surrounding the anus; m, male organ; rh, rh, rhinophores. × ⅔.
