lOI 



warmth and moisture that i>. provided they are hable 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 Sis-ails (6),^ belong to the class of the 

 Zvlollusca, 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 

 a.nd Belemnites. Both Slugs and Snails differ from other 

 Alolluscs 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 ?\Iolluscs this pulmonary chamber is almost always 

 absent, respiration taking place by other means. 



Snails or Hehcidae are provided with an external 

 spiral shell into which the animal can withdraw itself, and 

 there are three species which are commonly met with. The 

 Garden Snail (Helix asft!rsa, ]\Iull.) is the largest and 

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

 enough known to need no description, being easily recog- 

 nisable by its browm shell marked with pale irregular 

 lines. The Strawberry Snail [H. rnlescens, Pen.) has a 

 shell which seldom exceeds half an inch m diameter and 

 is more flattened m 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 neinoralis) has an extremely variable shell being 

 white, grey, pinkish, yellow or brown, and is marked w^th 

 one to five or more conspicuous browm spiral bands. It 

 is, moreover, considerably larger than the Strawberry 

 Snail. 



Slugs or Limacid^ 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 

 iniurious species have the shell m the latter position. The 

 situation of the shell is clearly marked externally and 

 the area of skm 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 



