_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] [Illustration: FIG. 66.--_Valvata piscinalis_ Müll.: _br_, branchia; _fi_, filament; _f.l_, foot lobes. (After Boutan.)] [Illustration: FIG. 67.--_Doris_ (_Archidoris_) _tuberculata_ L., Britain: _a_, anus; _br_, branchiae, surrounding the anus; _m_, male organ; _rh_, _rh_, rhinophores. × ⅔.] [Illustration: FIG. 68.--_Pleurophyllidia lineata_ Otto, Mediterranean: _a_, anus; _br_, secondary branchiae; _m_, mouth; _s.o_, sexual orifice.] Certain of the Nudibranchiata possess no special breathing organs, and probably respire through the skin (_Elysia_, _Limapontia_, _Cenia_, _Phyllirrhoë_). The majority, however, have developed secondary branchiae, in the form of prominent lobes or leaf-like processes (the _cerata_), which are carried upon the back, without any means of protection. These cerata are, as a rule, of extreme beauty and variety of form, consisting sometimes of long whip-like tentaculae, in other cases of arborescent plumes of fern-like leafage, in others of curious bead-like appendages of every imaginable shape and colour. In _Doris_ they lie at the posterior end of the body, in a sort of rosette, which is generally capable of retraction into a chamber. In _Phyllidia_ and _Pleurophyllidia_ these secondary branchiae lie, as in _Patella_, on the lateral portions of the mantle. The Scaphopoda in all probability possess neither true nor secondary branchiae. _Pulmonata._--When we use the term ‘lung,’ it must be remembered that this organ in the Mollusca does not correspond, morphologically, with the spongy, cellular lung of vertebrates; it simply performs the same functions. The ‘lung,’ in the Mollusca, is a pouch or cavity, lined with blood-vessels which are disposed over its vaulted surface in various patterns of network. The pulmonary sac or cavity is therefore a better name by which to denote this organ. [Illustration: FIG. 69.--_Geomalacus maculosus_ Allm., S. Ireland: =P.O=, pulmonary orifice.]