CHAP. VI MODES OF RESPIRATION I 5 I 



will perform their functions 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 jvidged from the fact that the 

 primary snbdivision of the Cephalopoda into Dibranchiata and 

 Tetrabranchiata is leased npon the number of branchiae they 

 ]^osses8. Fnrther, the three great divisions of the Gasteropoda 

 have been named from the position or character of the breathing 

 apparatns, viz. Prosobranchiata, Opisthobranchiata and Pulmonata, 

 while the name Pelecypoda has hardly yet dispossessed Lamelli- 

 branchiata, the more familiar name of the bivalves. 



Eespiration may be conducted by means of — («) Branchiae or 

 (Jills, (&) a Lung or Lung cavity, (c) 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), respira- 

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

 lung, whence the name Pulmonata. The land operculates 

 (Cyclophoridae, Cyclostomatidae, 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 

 Ampullaria, 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 he regarded as in process of change from an 

 aqueous to a terrestrial life, and in Siphonaria the branchia 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 only a 



^ 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 under- 

 gone (see p. 20), a transition from a marine to a terrestrial life. It Avas found that 

 marked specimens upon the rocks did not move their position for thirty-one successive 

 days (Pioc. Livcrp. Biol. Soc. iv. 1890, p. 50). 



- Diminutive of \-Tet ?, a comb. 



