354 EVOLUTION AND ANIMAL LIFE 



plasm begin to break up by repeated division (the parts all 

 being held together, however, in the wall of the zygote), and 

 by the end of the twelfth or fourteenth day the zygote's proto- 

 plasm may have become divided into ten thousand minute 

 sporozoites. The zygote wall now breaks down, thus releasing 

 the thousands of active little sporozoites into the general body 

 cavity of the mosquito. This cavity is filled with flowing 

 blood plasm — insects do not have a closed but an almost com- 

 pletely open circulatory system — and swimming about in this 

 plasm the sporozoites soon make their way forward and into 

 the salivary glands of the moscjuito. Now when the insect 

 pierces a human being to suck blood, it injects a certain amount 

 of salivary fluid into the wound (presumably to keep the blood 

 from clotting at the puncture) and with this fluid go many of 

 the sporozoites. Thus a new infection of malaria is made. 

 The sporozoites may lie in the salivar}^ glands for several weeks, 

 and so for the whole time from twelve to fourteen days after 

 the mosquito has become infected with the malarial parasite 

 by sucking blood from a malarial patient until the sporozoites 

 in the salivary glands finally die, it is a means of the dissemina- 

 tion of the disease. There can be no malaria without mos- 

 quitoes to propagate and disseminate it, and yet no mosquitoes 

 can propagate and disseminate malaria without having access 

 to malarial patients. The only mosquito species in this country 

 which has been proved to be a malaria disseminator is Anoph- 

 eles viaculijpennis , a spotted- winged form spread OA^er the 

 whole continent.- 



In the great branch or phylum of flatworms (Platyhelmin- 

 thes), that group of animals which of all the principal animal 

 groups is widest in its distribution, perhaps a majority of the 

 species are parasites. Instead of being the exception, the 

 parasitic life is the rule among these w^orms. Of the three 

 classes into w^iich the flatworms are divided, almost all of the 

 members of two of the classes are parasites. The common 

 tapeworm (Tcenia) (Fig. 212), which lives parasitically in the 

 intestine of man, is a good example of one of these classes. 

 It has the form of a narrow ribbon, which may attain the 

 length of several yards, attached at one end to the w^all of the 

 intestine, the remainder hanging freely in the interior. Its 

 body is composed of segments or serially arranged parts, of 

 which there are about eight hundred and fifty altogether. It 



