262 THE SPOROZOA 



(2) Life-history, It is probable that an alternation of genera- 

 tions, of schizogony and sporogony, occurs in all Haemosporidia, and 

 that there are no forms in which the schizogony is non-existent or 

 suppressed, as in Benedenia amongst Coccidia ; though there are 

 many in which the sporogony has not yet been described. The 

 most salient feature in which the life -cycles of different forms 

 differ from one another is the mode of infection ; that is to say, 

 with regard to the presence or absence of an intermediate host, in 

 which the sporogony takes place, and which serves to disseminate 

 the parasite. The brilliant investigations of Ross upon the Haemo- 

 sporidia of birds first demonstrated the agency of blood-sucking 

 gnats of the genus Culex in spreading the infection amongst avian 

 hosts, and the organised researches of Grassi and his Italian fellow- 

 workers have proved incontestably the part played by other mosquitos 

 of the genus Anopheles in carrying involuntarily the malarial germs 

 from one human being to another. In a similar way it has been 

 proved experimentally that the parasite of the Texas cattle-fever, 

 Piroplasma bigeminum, is transmitted from one ox to another by 

 ticks (Ehipicephalus annulatus = Boophilus bovis); but in this case the 

 part played by the intermediate (invertebrate) host is much more 

 complicated than in the infection of birds or man with malaria 

 by gnats, since the parasite passes through two generations of ticks. 



The ticks which nourish themselves upon cattle and other mammals 

 become sexually mature at their last moult. They then pair, and the 

 fertilised females, after gorging themselves with blood, drop off on to the 

 ground. Each female then lays about 2000 eggs, and within the shell 

 of each egg a large quantity of blood is deposited, to serve as vitellus for 

 the developing embryo. When oviposition is completed, the female 

 shrivels up, and becomes a dried, empty, lifeless skin. From the egg 

 is hatched a larva, which has only three pairs of legs, and contains in 

 its abdomen a certain quantity of blood, the still unabsorbed remains 

 of its share of its mother's last meal. The newly-hatched larva crawls 

 on to a blade of grass or other convenient coign of vantage, from which 

 it either passes on to the skin of a fresh host, or drops off dead from 

 starvation, if no favourable opportunity occurs for changing its situation 

 before its supply of blood is exhausted. 



A remarkable fact, with reference to the transmission of Texas-fever, 

 was first demonstrated experimentally by Smith and Kilborne, and subse- 

 quently confirmed by Koch [70] and other observers, namely, that if the 

 mother-tick drew its supply of blood from an ox infected with Piroplasma, 

 her progeny are born into the world infected with the parasite, and become 

 the means of disseminating the disease amongst healthy cattle. Thus is 

 explained the long incubation -period of the disease, the time required 

 for it to spread from diseased to healthy cattle being about forty -five to 

 sixty days ; of this thirty days are taken up by the development of the 

 egg of the tick, the remainder probably by the development of the 

 parasite within the ox (Smith [97]). 



