198 PARASITISM 



belonging chiefly to the trypanosome and spirochete group are capable 

 of passing through the finest filters, and such forms of protozoa, if any, 

 might be expected to infect an embryo in utero. This is certainly 

 true of the organism of human syphilis, congenital cases not infre- 

 quently occurring in which the parasites are transmitted either with 

 the spermatozoa or with the egg, or through the placenta from the 

 mother infected during pregnancy. Such congenital cases are often 

 highly virulent; all organs and tissues of the unfortunate infant may 

 be over-run with the malignant spirals (Fig. 83). 



With transmission by contact or by inheritance, there is, strictly 

 speaking, no free or external life of the parasite, the organisms passing 

 directly from one living host into another, and this form of infection is 

 often bound up with one of the most interesting and important of the 

 protozoan vital phenomena, the transmission by intermediate hosts. 



(d) Transmission by Intermediate Hosts. — Direct infection by way 

 of the digestive tract by ingestion of spores of the parasites with food 

 may become complicated by passive carriage through intermediate 

 hosts often of a quondam character. While not proved, this appears 

 to be a highly probable means of infection. Thus, as Minchin points 

 out, in the case of the monocystis parasites of the earthworm, where 

 the organisms are parasitic in the seminal vesicles of the worm, there 

 is but slight possibility of the parasite spores passing to the outside 

 with the spermatozoa or through the dorsal pores of the worm, and 

 there is little doubt that the animals are infected by way of the diges- 

 tive tract. It is suggested by Minchin that the infected worms are 

 eaten by birds, and that the spores of the gregarine, protected by their 

 resistant coatings, pass undissolved through the avian digestive tract, 

 to be disseminated with the bird's feces about the ground, where in 

 time they may be again eaten by a worm. Similar conjectures might 

 be made for other animals whose habits, life histories, and parasites 

 are known. 



A mode of transmission such as this would involve only a passive 

 phase in the life history of the protozoan parasite; in the majority of 

 cases where the relation of parasites to intermediate hosts are fully 

 made out the period in such a host involves some of the most impor- 

 tant activities in the life of the parasite. Here are to be found some of 

 the most perfect adaptations of means to ends that are known in 

 biology; those forms which are not protected by resistant coverings 

 and where infection is brought about through the aid of an obligatory 

 intermediate host are the most remarkable. The malaria organisms, 

 for example, if sucked with the blood into the digestive tract of a 

 mosquito of the genus anopheles, are all digested save the conjugating 

 forms, which are apparently endowed with some greater power of 

 resistance than are the vegetative forms. But if the same parasites are 

 taken into the stomach of a mosquito of the genus culex, gametes, and 



