28 Our British Snails 



contained in 145 transverse rows. The organs 

 of digestion are complex and practically much 

 the same as our own. Little vegetation would 

 be left in nature had not, on the one hand, snails 

 been kept down by many enemies as well as by 

 their need of hibernation and their short life ; 

 while on the other by numerous devices in the 

 course of ages many plants have protected 

 themselves against the moving machine of a 

 snail's mouth. Cultivated plants, which 

 generally lose their natural protections, have to 

 be guarded by human guards or gardeners. 

 Some plants defend themselves by prickles or 

 hairs, some by hardening themselves with lime 

 or flint, some by bitter or acrid juices. A heart of 

 two chambers, veins, arteries, and blood our snail 

 possesses, and, like man, the old snail has a slower 

 pulse than th^ young one, and in both exercise 

 increases the pulse rate and also warmth. 

 Breathing is accomplished by a single chamber 

 or air-cell, but also through the skin. As in the 

 case of plants, some kinds are male and female 

 separately, and as some have both powers and 

 products in the same plant, so also is it with 

 mollusca. H. aspersa and most Gasteropoda 

 are of the latter kind. 



Having now taken H. aspersa as the repre- 

 sentative of our univalves, let us take the " Pearl 

 Mussel " — Unio margariiifer — as that of our bi- 

 valves, all of which live in the water, whereas 



