ORGANIC RESPONSE 



281 



that it is very difficult to discern the Caprella on the 



hydroid colony even when one is looking for them. 



To the safety afforded by this protective resemblance 



may be added the protection of the stinging nettle 



cells of the hydroid. Such a relation, which profits 



but one party 



to the associa- 



tion and on the 



other hand does 



not injure the 



other, is termed 



mutualism or 



commensalism. 



In other, cases 



both sides profit 



by the connec- 



tion. A very 



familiar exam- 



ple is the rela- 



tion that exists 



between the 



aphids and 



ants. The 



former secrete 



Q iolKr lil* pnVk 



a 3eny-iiKe su 



stance called 



" honey-dew," which the ants are very fond of, and 



in order to maintain a supply, the ants keep the 



aphids in flocks, as men do cattle, tending them, 



carrying them back if they stray, and " milking " 



them by stroking them with their antennae, thereby 



FlG - 10 - R se A Phids visited by ants, natural 



size from life _ (From Kel]ogg j 



