PROTECTION, SUPPORT, ETC. 141 



The connecting system consists simply of various tissues and 

 substances by which the different organs are bound firmly to- 

 gether. These can only be seen upon microscopical examination. 

 The most important of them is known as connective tissue. 



As to protective structures, the earthworm is probably one of 

 the most defenceless of animals. Nevertheless there are certain 

 structures which are clearly for this purpose. The cuticle which 

 covers the surface is a thin but tough membrane which pro- 

 tects the delicate skin from direct contact with hard objects. It 

 passes into the mouth and anus and lines the entire alimentary 

 canal. In the gizzard, where food is ground up, it is prodigiously 

 thick and tough, and must form a very effective protection for 

 the soft tissues beneath it. The main defence of the animal lies, 

 however, not in any special armor, but in those instincts which 

 lead it to lie hidden in the earth during the day and to venture 

 forth only in the comparative safety of darkness. 



