210 ZOOLOGY 



study their reactions to different stimuli, taken one at a time. 

 In this chapter we shall consider the different sorts of animals 

 related to the slugs and snails, with special reference to their 

 behavior. 



The slug Ix'longs to the group of Mollus'ca' defined as 

 animals which possess two distincti^'e organs — the foot, by 

 means of which locomotion is effected, antl the mantle, a fold 

 of skin covering over or enclosing a greater or less part of the 

 body. The mantle usually secretes on its outer surface a cal- 

 careous shell. Exceptionally, both shell and mantle may be 

 entirely absent in the adult. Among the Mollusca the slug 

 is placed in the class Gasteropoda. The front part of the body, 

 called the head, bears the sense-organs which are in nervous 

 connection with a brain situated jiist over the mouth. The 

 foot is an unpaired organ situated upon the ventral side of the 

 body and is used to crawl upon. It is rather remarkal^le that, 

 despite tlic slowness of its locomotion and the ease witli which 

 the movements associated with locomotion can be studied, the 

 exact mechanism of locomotion in the slug is unknown. The 

 slug exudes a constant stream of mucus from the mouth of a 

 sack situated just over the sole of the foot, and this stream of 

 mucus seems to be the apparatus of propulsion. The stream 

 of new mucus impinges upon that already thrown out and the 

 reaction drives the animal forward. Gastero]Dods - usually 

 have an external shell, but some species are without it, as 

 is the case with the slug. When the shell is present, it is made 

 of one piece; that is, it is univalve. 



Slugs l)elong to the grouj) of air-breathing, land-inhabiting 

 gasteropods, or Pulmonata.' They may hv found in the spring, 



^mollis, soft. '' gastcr, belly, stomarh ; pons, toot. 



■' Provided with lung,s ; from pulmo, lung. 



