542 BIOLOGY OF THE PROTOZOA 



of the microgametes, and cellular processes are generally absent. 

 The endoplasm is usually well stored with products of metabolism, 

 some of which are so characteristic that they have received the name 

 of coccidin. They are all osmotic in nutrition, and infection is 

 always, so far as known, by the contaminative method through 

 the digestive tract. The sporozoite penetrates an epithelial or other 

 definitive cell, grows at the expense of the cell which it ultimately 

 destroys, and forms agametes while still intracellular. Cyclospora 

 karyolytica Schaudinn of the ground mole enters the nucleus of the 

 intestinal epithelial cell and as a karyozoic parasite completes its 

 life history. 



Sub-order 2. Hemosporidia Danilewsky, em. Dofleix. 



The Hemosporidia are Coccidia-like forms specifically adapted 

 for parasitic life in the blood, particularly of the erythrocytes, 

 although some forms become intracellular parasites of the inner 

 organs. Vertebrates of all classes— mammals, birds, reptiles, 

 amphibia and fish— are subject to infection by one type or other 

 and man is particularly susceptible, the malarial organisms causing 

 serious human diseases which in the tropics are frequently fatal. 



Hemosporidia are minute forms, particularly in the agamous 

 stages during which they frequently show highly motile ameboid 

 stages, but in other cases they are more rigid and appear like the 

 hemogregarines. Contractile vacuoles are absent but cytoplasmic 

 non-contractile vacuoles, probably connected with nutrition, are 

 characteristic. Pigmented granules (Melanin) are also character- 

 istic and are formed as a product of hemoglobin break-down and 

 liberated only at periods of reproduction. Other products of 

 metabolism, in the form of toxins, may be liberated at the same 

 time. 



Alternation of asexual and sexual generations is the rule, the 

 former taking place in the blood of vertebrates, the latter in the 

 digestive tract of some blood-sucking arthropod, insects in particu- 

 lar. The prevailing opinion is that arthropods were the primary 

 hosts and that parasitism in the blood is the result of adaptation. 

 One such adaptation, and a very essential one, is the absence of 

 protective capsules about the sporozoites. The latter are always 

 formed in the primary or invertebrate host and are transmitted to 

 the vertebrates at the time of drawing blood. A sporozoite pene- 

 trates an erythrocyte and grows to an agamont which forms mul- 

 tiple agametes after a definite interval; these agametes are liberated 

 into the blood where other erythrocytes are entered and the asexual 

 cycle is repeated. The parasites thus multiply rapidly by geometri- 

 cal progression until enough blood elements are destroyed to pro- 

 duce the first marked symptoms of the infection. Hegner and 



