INSECTS AND OTHER ORGANISMS 413 



Sporozoa, to which belong the Gregarinida previously 

 mentioned (p. 409), are perhaps the most remarkable 

 because they display in the order Haemosporidia a specialised 

 and complex adaptation to two alternate hosts — a blood- 

 sucking insect (or other Arthropod such as a tick) and a 

 vertebrate animal, on which that insect feeds. Many birds 

 have Haemosporidia in their blood, each parasite when 

 young invading a red corpuscle, growing and feeding therein 

 until the blood-cell is broken down and the haemosporidian 

 is set free in the plasma to divide into a number of daughter- 

 cells, each of which can in its turn invade a fresh red 

 corpuscle. After a number of such generations due to the 

 division of the parent, individual parasites, free in the 

 plasma, give rise to slender flagellate forms, while others 

 assume a spherical shape and a passive habit. Further 

 development can proceed only in the stomach of a gnat 

 which happens to suck blood from the infected bird ; in 

 the insect's digestive tract the flagellate cells (which are 

 really sperms) fertiUse the spherical ones (which are eggs), 

 and the resulting zygote becomes elongate or '* vermicular " 

 and penetrates into the stomach-wall where it greatly 

 enlarges and undergoes extensive division so that ultimately 

 a spherical cyst, filled with minute spindle-shaped motile 

 cells or sporozorites, projects into the gnat's body-cavity. 

 The sporozoites break out from the ruptured cyst and enter 

 the salivary glands whence they may be injected into the 

 blood of some other bird of which the gnat may make use 

 to secure a meal. Thus there is a sexual phase of the 

 parasite's life-cycle in the gnat, leading up to the production 

 of multitudinous sporozoites, each of which, injected into the 

 blood of a suitable bird-host can invade a red corpuscle 

 and begin a series of asexual generations reproducing by 

 multiple division (" schizogony "). It is noteworthy that 

 the Sporozoa thus transmitted by alternate insect hosts 

 have no firm-coated, resistant spores, such as in members 

 of the class generally protect the sporozites until they have 

 been swallowed by the proper host. In these Haemsporidia, 

 the active sporozites (see Fig. 84, b) are passed into the 



