30 ANIMAL STUDIES 



garina. In some species there are fine ducts or canals 

 leading from the center of the cyst through the wall to the 

 outside, and through these canals the spores issue. Some- 

 times two Gregarince come together before encystation and 

 become inclosed in a common wall, the two thus forming a 

 single cyst. This is a kind of conjugation. In some spe- 

 cies each of the young or new Gregarinm coming from the 

 spores immediately divides by fission to form two indi- 

 viduals. 



Eelated to the GregarincB are those minute protozoan 

 parasites which live in the blood-corpuscles of man and some 

 of the lower animals, and are called Hoematozoa. Three 

 species of these, living in the blood of man, cause the three 

 kinds of malarial fever, known as tertian, quartan, and 

 remittent. These malarial Hcematozoa, known generally 

 as Hcemamcela,, can multiply by asexual sporulation in the 

 blood, but produce also certain sexual individuals, which, 

 when taken into the stomach of a mosquito which has 

 sucked blood from a malarial patient, give rise to a zygote 

 which encysts in the outer walls of the stomach, and breaks 

 up into numerous blasts or embryos, which escape into the 

 blood of the mosquito, and thence to all parts of its body, 

 and especially to the salivary or poison glands. When now 

 this infected mosquito pierces the skin of another man, 

 and pours into the wound, as it regularly does, a quantity 

 of saliva, numbers of larval Hcemamcebcz also enter the 

 blood, and, multiplying here, soon set up the disease 

 malaria in the bitten person. It has been definitely proved 

 that malaria is thus disseminated by mosquitoes, and it is 

 highly probable that it is contracted in no other way. 



30. Characteristics common to the Protozoa. We have 

 now studied the principal structures which serve in loco- 

 motion among these simple one-celled forms, also the means 

 by which they catch their food, and we shall now glance at 

 the internal processes, which are much the same in all. 



After the food has been taken into the cell, it is proba- 



