l60 BREATHING ORGANS IN PULMONATA chap. 



The Scaphopoda in all probability possess neither true nor 

 secondary branchiae. 



Pulmonata. — When we use the term ' lung,' it must be re- 

 membered 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. 



It seems probable, as has been already shown (pp. 18-22), that 

 all Pulmonata are ultimately derived from marine forms which 

 breathed water by means of branchiae. Thus we find inter- 

 mediate forms, such as Siphonaria^ possessed of both a branchia 

 and a pulmonary sac, the former being evanescent, while in 

 Gradinia and Amphihola it has quite disappeared. In tlie vast 



Fig. 69. — Geomalacus niaculosus Allm., S. Ireland: P.O, pulmonary oritice. 



majority of Pulmonata no trace of a branchia remains ; its func- 

 tion is performed by a chamber, always situated at the right 

 side of the animal, and generally more or less anterior, admit- 

 ting air by a narrow aperture which is rhythmically opened and 

 closed. In Avion and G-eomalacus (Fig. 69) this aperture is in 

 the front of the right side of the ' shield,' in Limax (Fig. 71) in 

 the hinder part, in TestaceUa (Fig. 20) it is near the extremity 

 of the tail, under the spire of the shell ; in Janella it is on the 

 middle of the right edge of the shield (Fig. 70). If a specimen 

 of Helix aspersa, or better, of H. pomatia, is held up to the light, 

 the beautiful arborescent vessels, with which the upper part of 

 the pulmonary chamber is furnished, can be clearly seen by 

 looking through the aperture as it dilates. It is only in the 

 Auriculidae that an actual spongy mass of lung material appears 

 to exist. When in motion, a ITelix inspires air much more fre- 

 quently than when at rest. Temperature, too, seems to affect 

 the number of inspirations ; it appears doubtful whether, during 



