204 THE ZOOLOGIST. 



constant clearing out of all ditches and dykes and pools would, of 

 course, be most beneficial, especially if the weeds and mud are 

 burnt, so that the eggs of the snails are destroyed, or lime and 

 salt sprinkled over the mud, so as to kill those of truncatulus as 

 well as the snails themselves, and any escaped embryos of the 

 flukes. Beyond this little else seems practicable. 



Slugs {Limacidcz) and Snails (Helicid<z). — Snails and slugs 



are great pests to the gardener, and every now and then a 



plague of one or the other makes its appearance and attacks 



our field crops. Both snails and slugs possess a head which 



bears tentacles, and also a pair of eyes which may be borne at the 



tip of these tentacles. The foot is flattened. Snails possess lips, 



but the organ they use for destroying plant-tissues is the curious 



swollen rasping tongue, the " radula," the surface of which is 



covered by rows of variously arranged teeth. They breathe by 



means of the highly vascular inner walls of the mantle-cavity. 



Snails and slugs are hermaphrodite. The eggs are laid in batches 



in the ground and under stones. The injurious snails belong 



chiefly to the genus Helix. Almost every wood and hedge, field 



and garden yields some kind of Helix; others are partial to the 



sands near the sea, water-courses, and damp places. Their habits 



are nocturnal and crepuscular, and are seldom seen crawling in 



the daytime, unless after heavy rains. This latter habit has given 



rise to the popular idea that the occasional snail-plagues come in 



the rain-clouds. As soon as the sun shines they crawl to some 



shelter — under stones, moss, or beneath the leaves of the plants 



they attack. When the breeding season is on, the male organs of 



each snail are supplemented by one or more curious crystalline 



darts, which they thrust out at one another : these curious 



structures are found in special sacs (the so-called "dart-sacs"), 



and are peculiar to the genus Helix. The eggs are generally 



round, white, semi-transparent bodies, and are always laid in 



batches in slanting galleries under ground formed by the 



"mother" snail. The slugs {Limacidce), unlike the snails, have 



only a rudimentary shell, or an indefinitely formed one placed 



under the mantle. They mostly frequent damp and shady places, 



and during daytime they bury themselves in tunnels under the 



earth. Unlike the snails, they generally deposit their eggs singly 



under the ground, and the eggs are very numerous. The two 



chief genera are Avion and Limax, the former being distinguished 



