l6o BREATHING ORGANS IN PULMONATA chap. 



The Scaphopoda in all probability possess neither true nor 

 secondary branchiae. 



Pidmonata. — When we use the term ' hmg,' it must be 

 remembered that this organ in the Mollusca does not correspond, 

 morpliologicall}', with the spongy, ceHular 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 hy which to denote this organ. 



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

 that all Pulmonata are ultimately derived from marine forms 

 which l)reathed water by means of branchiae. Thus we find inter- 

 mediate forms, such as Svphonaria, possessed of both a branchia 

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

 Gndin'ui and Aiivpliihohi it has (piite disappeared. In the vast 



Fio. 6P. — Oemvi'ilaciif inaculosns Alliu., S. Ireland : P.O, pulnioiiary ovificc. 



majority oi Pulmonata no trace of a branchia remains ; its function 

 is performed by a chamber, always situated at the right side of 

 the animal, and generally more or less anterior, admitting air by 

 a narrow aperture which is rhythmically opened and closed. In 

 Arion and Geomalacus (Fig. 69) this aperture is in the front of the 

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

 in Testacella (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. j^omatia, is held up to the light, the beautiful 

 arborescent vessels, with which the upper part of the pulmonary 

 chamljer is furnished, can Ijc 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 Helix inspires air much more frequently than when 

 at rest. Temperature, too, seems to affect the niunber of inspira- 

 tions ; it appears doubtful whether, during hibernation, a snail 



