CHAP. VI MODES OF RESPIRATION 1 5 I 



if they can manage to retain an extremely small amount of 

 moisture.^ 



The important part which the respiratory organs play in the 

 economy of the Mollusca may be judged from the fact that the 

 primary subdivision of the Cephalopoda into Dibranchiata and 

 Tetrabranchiata is based upon the number of branchiae they 

 possess. Further, the three great divisions of the Gasteropoda 

 have been named from the position or character of the breathing 

 apparatus, viz. Prosobranchiata, Opisthobranchiata and Pul- 

 monata, while the name Pelecypoda has hardly yet dispossessed 

 Lamellibranchiata, the more familiar name of the bivalves. 



Respiration may be conducted by means of — (a) Branchiae 

 or Gills, (6) a Lung or Lung-cavity, (^) the outer skin. 



In the Pelecypoda, Cephalopoda, Scaphopoda, and the great 

 majority of the Gasteropoda, respiration is by means of branchiae, 

 also known as ctenidia? when they represent the primitive Mol- 

 luscan gill and are not ' secondary ' branchiae (pp. 156, 159). 



In all non-operculate land and fresh-water Mollusca, in the 

 Auriculidae, and in one aberrant operculate (Amphihola)^ res- 

 piration is conducted by means of a lung-cavity, or rarely by a 

 true lung, whence the name Pulmonata. The land operculates 

 (Cyclophoridae, C3^clostomatidae, Aciculidae, and Helicinidae) 

 also breathe air, but are not classified as Pulmonata, since other 

 points in their organisation relate them more closely to the 

 marine Prosobranchiata. Both methods of respiration are united 

 in AmpuUaria, which breathes indifferently air through a long 

 siphon which it can elevate above the surface of the water, and 

 water through a branchia (see p. 158). Siphonaria (Fig. 57) is 

 also furnished with a lung-cavity as well as a branchia. Both 

 these genera may be regarded as in process of change from an 

 aqueous to a terrestrial life, and in Siphonaria the bi-anchia is to 

 a great extent atrophied, since the animal is out of the water, on 

 the average, twenty-two hours out of the twenty-four. In the 

 allied genus Gadinia, where there is no trace of a branchia, but 



1 The result of some experiments by Professor Herdman upon Littorina 

 rudis, tends to show that it can live much better in air than in water, and goes 

 far to support the view that the species may be undergoing, as we know many 

 species must have undergone (see p. 20), a transition from a marine to a terres- 

 trial life. It was found that marked specimens upon the rocks did not move 

 Their position for thirty-one successive days {Froc. Liverp. Biol. Soc. iv, 1890, 

 p. 50). 



- Diminutive of /creis, a comb. 



