384 THE PROTOZOA 



memoir showed that the parasite of Texas cattle-fever (Babesia 

 'boms or bigemina) was transmitted from sick to healthy oxen by 

 the agency of ticks. The method of transmission is of a peculiar 

 type, which finds its explanation in the habits and life-history of 

 ticks. These arachnids have typically three stages in their life- 

 history (1) the minute six-legged larva hatched from the egg, 

 which, after growing to its full size, sheds its skin and appears as 

 (2) the nymph, eight-legged, but sexually immature ; the nymph 

 after another moult becomes (3) the adult tick, sexually mature and 

 with four pairs of legs. In each of these three stages of the life- 

 history the tick feeds, as a rule, but once. Consequently, if the 

 parasites are taken up by the tick at one stage of its existence, they 

 cannot be reinoculated into another host until a later stage of the 

 tick. Smith and Kilborne found that the parasites taken up by the 

 adult female ticks passed through their ova into the next generation 

 of the ectoparasites, so that the minute larval ticks, progeny of an 

 infected mother, were the infective agents which spread the disease 

 amongst the cattle. 



Subsequent investigations have confirmed and extended the dis- 

 covery made by Smith and Kilborne, and in every case the in- 

 vertebrate host of any species of piro plasm appears to be a tick. 

 In P. bovis (bigeminum) the parasites develop only if taken up by 

 an adult female tick (Koch), but this is not so in other cases. The 

 parasites may be taken up by the tick at various stages, and returned 

 to the vertebrate host at a later one ; for instance, by the larva and 

 returned by the nymph, or by the nymph and returned by the adult, 

 or by the adult and returned by the larva of the next generation. 



Although the transmission of piroplasms by ticks is well established, the 

 developmental cycle of the parasite in the tick is known only in a fragmentary 

 and incomplete manner. The most complete accounts are those given by 

 Christophers (732) for Piroplasma canis, and Koch (743) for P. bovis, whose 

 observations supplement each other, since Koch studied chiefly the earlier 

 stages, while Christophers' investigations appear to be more complete for 

 later phases of development. Stages in the tick are also described by 

 Dschunkowsky and Luhs (734), but in a disconnected manner, and observa- 

 tions on the development in cultures have been published by Kleine (742) 

 and by Nuttall and Graham-Smith (750). Accounts differ chiefly as to the 

 events at the beginning of the development. So far as it is possible to make 

 a connected story out of the published observations, the development in the 

 tick appears to comprise six principal phases : 



1. The piroplasms taken up in the blood pass into the stomach of the 

 tick, and there the pear-shaped forms are set free from the corpuscles, 

 these forms alone being capable of further development. After 

 about twelve to eighteen hours they become amoeboid, sending out in all 

 directions slender, stiff, sharply-pointed pseudopodia which are slowly re- 

 tracted and emitted again. Usually the pseudopodia are given off chiefly 

 from the thicker end of the pear-shaped body, but in some cases the form is 

 spherical and the appearance of the parasite strikingly Heliozoon-like (Fig. 162, 

 A C). The nucleus of the parasite divides into two parts a larger mass, 



