ioi ,/ r; , v , 



warmth and moisture that is '^provided: they" W^l^Hi to 

 multiply exceedingly. Experiments carried out at the 

 Rothamsted Station have shown that we can very con- 

 siderably reduce this undesirable population by partial 

 sterilization of the soil by means of steam. In cases of 

 very bad infestations of Eelworm this method is said to 

 be the only effective remedy at present available. 



SLUGS and SNAILS (6), (7), belong to the class of the 

 Mollusca, which is a large assemblage of animals includ- 

 ing such divers forms as Oysters, Whelks, Scallops, Octopi, 

 and the familiar fossils which are known as Ammonites 

 and Belemnites. Both Slugs and Snails differ from other 

 Molluscs in being land and not aquatic animals. They 

 are always provided with a pulmonary chamber, which is 

 a kind of lung enabling them to breathe in the air. In 

 aquatic Molluscs this pulmonary chamber is almost always 

 absent, respiration taking place by other means. 



SNAILS or Helicidae are provided with an external 

 spiral shell into which the animal can withdraw itself, and 

 there are three species which a*re commonly met with. The 

 Garden Snail {Helix aspersa, Mull.) is the largest and 

 its shell measures about ii inches in diameter. It is well 

 enough known to need no description, being easily recog- 

 nisable by its brown shell marked with pale irregular 

 lines. The Strawberry Snail (//. rufescens, Pen.) has a 

 shell which seldom exceeds half an inch in diameter and 

 is more flattened in form. It also varies in colour from 

 dirty grey to brown or reddish-brown, often with a num- 

 ber of transverse streaks of a darker tint. The Wood 

 Snail (H. nemoralis} has an extremely variable shell being 

 white, grey, pinkish, yellow or brown, and is marked with 

 one to five or more conspicuous brown spiral bands. It 

 is, moreover, considerably larger than the Strawberry 

 Snail. 



SLUGS or Limacidae are naked and only possess a 

 vestigal shell, which is placed near the hinder end of the 

 body or buried beneath the skin of the back; all the 

 injurious species have the shell in the latter position. The 

 situation of the shell is clearly marked externally and 

 the area of skin covering it is known as the shield or 

 mantle. Closely related to the margin of the latter, on 

 the right side of the body, is the respiratory pore a well- 

 defined aperture leading into the pulmonary chamber. 

 Slugs secrete an abundance of mucous, which serves to 



