CHAPTER XVIII 



DEFENCE 



Most of the diseases to which animals are liable are due 

 to the invasion of the body by micro-organisms. It is 

 famihar to every one that when an epidemic occurs, of the 

 many who are exposed to the infection, not all take the 

 disease ; some are naturally or innately immune. Of those 

 who take the disease a number, in most cases the greater 

 number, recover; those who recover are for a certain 

 period or for ever insusceptible to the disease. They have 

 acquired immunity. 



The micro-organisms owe their effects to the toxins or 

 poisons which they produce. Immunity, whether natural 

 or acquired, consists in the prevention of the propagation 

 of the organisms, the neutrahsation and excretion of their 

 toxins, and the repair of damaged tissue. 



The methods by which the animal overcomes the action 

 of bacteria are seen at their simplest in lower forms of hfe. 

 Unicellular organisms, such as Amoeba and Paramoecium, 

 hve upon bacteria. They ingest them and subject them to 

 the hydrolysing action of their proteolytic enzymes. On 

 this account Amoeba and Paramoecium are practically 

 immune to bacteria. 



If we now pass to the simpler multicellular animals we 

 find a reaction of a shghtly more complicated kind. Let 

 us take the developing starfish in the Gastrula stage. 

 The body consists of an invaginated cup, the outer layer 

 being the ectoderm, the inner the endoderm. Between 

 them is the body-cavity in which float free mesoblastic 



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