PARASITISM 323 



During many years in the past those which did not possess 

 protective features were killed by the ever-pursuing ene- 

 mies, while the survivors transmitted to their offspring 

 their special protective characteristics. 



23. PARASITISM 



Symbiosis. There are certain species of animals which 

 are always found in association with certain other species, 

 because in the struggle for existence they are mutually 

 helpful. The ants are often to be seen in company with 

 the plant lice which feed on the sap in the tender twigs and 

 leaves. From the alimentary canal of the plant lice exudes 

 a sweet fluid much sought after by the ants. The ants 

 assist the lice by transporting them to the tenderest parts 

 of the plant, and in some cases during inclement weather 

 even remove them to places of shelter. This condition 

 of intimate association for mutual benefit is known as 

 symbiosis. Some of the crabs bear on their back sponges 

 which render them less easily visible to their enemies, and 

 the crab repays the kindness by carrying the sponges to 

 good feeding-grounds. 



True Parasites. A still closer association between 

 species, where only one derives any benefit, represents a 

 condition designated parasitism. The animal which thrives 

 by feeding on the living blood or tissue of another called 

 the host is a parasite. Parasitic animals are found among 

 nearly all the subkingdoms, but they are the most numer- 

 ous among the protozoans, worms, and arthropods. Only 

 about a dozen species of vertebrate parasites are known. 

 They are the lamprey eels (Fig. 215) and hagfish, which 



