374 



ELEMENTARY BIOLOGY 



is illustrated by the skunk. This animal, as everyone knows, is 



capable of ejecting a foul-smelling liquid from a gland at the 



rear of the body when it is greatly agitated. 



Real fighting appears among animals that have mouths and 



appendages that are capable of grasping. These organs are at 



the same time food- 

 getting organs. Lob- 

 sters and crabs are 

 very pugnacious ani- 

 mals, or at least that 

 is the impression they 

 make upon the ob- 

 server. Most of the 

 mollusca (clams, oys- 

 ters, scallops, etc.) de- 

 pend on their armors 

 for defense against 

 possible aggressors ; 

 some of them, how- 

 ever, as the octopus, 

 are very good fighters 

 (Fig. 95)- 



Among the insects 

 many are predatory, 

 using their append- 



FIG. 195. The fall of a leaf 



A, leaf dropping off ; s, self-healing scar remaining on 

 twig; B, microscopic view of section through base of 

 leafstalk ; a, angle between base of stalk and twig. In 

 plants that regularly drop their leaves in the autumn 

 there is formed a special layer of cells in the stalk of 

 each leaf, and sometimes of each leaflet of a compound 

 leaf. These cells, s /, are thin-walled and turgid. Their 

 contents break down into a mucilaginous mass, which 

 dries up. A slight movement is now sufficient to break 

 the fibrovascular bundle at this point, and as the leaf is 

 removed the exposed surface becomes a self-healing scar 



ages (Fig. 191) or 

 their mouths (Fig. 192) in catching prey. But very few use 

 these organs in fighting their enemies. The colonial insects, 

 especially the ants, furnish the best examples of this mode of 

 protection (Fig. 193). The bees, wasps, and hornets fight 

 when they are disturbed or when the colony is disturbed, 

 but in fighting they use the sting (see Fig. 194), which has 

 nothing to do with food-getting or with locomotion. 



The horns of mammals are associated with the instinct to 

 defend or fight, and are quite independent of the organs or 



