BACKBONELESS ANIMALS WHICH BREATHE IN AIR 433 



the animal when it is uncovered by the tide. A Limpet, however, 

 possesses special gills all round the body (see p. 396), which are 

 used when the tide is up, and perhaps also breathe air when the 

 tide is down. It is practically certain that some of the Land-Snails 

 have sprung from ancestors which lived, like Limpets, between 

 tide-marks, and were able to breathe both air dissolved in water 

 and ordinary atmospheric air. Driven by keen competition from 

 the shore, the descendants of these forms took to living altogether 

 on dry land, and gave up breathing air dissolved in water, while 

 at the same time their gill-cavities became specialized as air- 

 breathing organs. Other Land-Snails probably took origin from 

 estuarine or freshwater species, in which cases the still earlier 

 ancestors were doubtless marine in habit. 



Examination of a Garden-Snail {^Helix aspersa), a common 

 and very typical Pulmonate, shows the presence of a cavity 

 opening above the neck, corresponding 

 precisely to the gill-cavity of a Limpet 

 both as to position and in regard to the 

 complete absence of gills. This "lung", 

 as it may be called for convenience 

 sake, is of large size, and the inner sur- 

 face of its thin roof (the mantle) is 

 raised into a net-work of ridges (fig. 

 555) by which the surface exposed to 

 the air is increased in area (compare 

 p. 424). The muscular floor of the 

 lung is curved, and by its movements 

 brings about the passage of air into 

 and out of the breathing cavity. The 

 lung does not open in front by a wide 

 slit, as did the ancestral gill-cavity to 

 which it corresponds, but by a small 

 round hole on the right-hand side. 

 This narrowing of the external aper- 

 ture is in order to prevent the lung 

 from drying up, which would prevent 



it from performing its function, and by muscular action the com- 

 paratively small opening can be varied in size or even closed 

 altogether according to circumstances. 



Land-Slugs may be broadly described as flattened-out Snails, 



I^ig- 555— Roof of Lung of Land-Snail 

 [Helix), showing inner surface (enlarged). 

 Impure blood flows to the net-work, of ridges 

 and, when purified, is carried by the pul- 

 monary vein (pul.vein) into the auricle (au) 

 of the heart and thence to the ventricle (vn), 

 which pumps it to the body; UR., ureter 

 carrying waste from kidney and opening in 

 front (upper end of figure) close to end of 

 rectum. 



60 



