THE MODES OF LIFE OF THE PROTOZOA 23 



an effect often produced by tissue-parasites, such as the Myxobolidce, 

 or by species of Microsporidia. In such cases also the parasite is 

 pathogenic to its host, and frequently lethal. 



3. The parasite remains in the host until the latter is eaten by 

 some animal which preys upon it. The propagative phases of the 

 parasite are able, however, to resist digestion by the animal that 

 has devoured their former host, and pass unaltered through its 

 intestine, to be finally cast out with the dejecta. This is almost 

 certainty the method by which the common Monocystis of the earth- 

 worm infects its host. The parasite produces resistant spores in 

 the worm ; the worm is eaten by a bird, mole, frog, OF some other 

 animal, through the digestive tract of which the spores pass un- 

 altered ; they are scattered abroad with the faeces, and may then 

 be swallowed by another earthworm, in which they germinate and 

 produce an infection. 



4. As in the last case, the host, together with its parasites, is 

 devoured by some animal, in which, however, the parasite is not 

 merely carried passively, but again becomes actively parasitic. 

 Hence in this case there is an alternation of -hosts, one of the two 

 hosts becoming infected by devouring the other. This mode of 

 infection, which is well known to occur commonly among parasitic 

 worms, such as Cestodes, is probably also frequent among Pro- 

 tozoa ; but at present only two cases of it are known with certainty. 

 One is that of the species of the genus Aggregata (vide infra, p. 353), 

 parasites of crabs and cephalopods, such as the cuttle-fish and the 

 octopus. In the cephalopod the parasite forms resistant spores 

 which pass out with the faeces, and may then be devoured by crabs. 

 In the crab the spores germinate and give rise to a second form of 

 the parasite, which lives and multiplies in its new host. If, as fre- 

 quently happens, the crab is eaten by a cephalopod, the parasite 

 completes its life-cycle by becoming once more a parasite of the 

 cephalopod. Another case is that of Hcemogregarina muris in the 

 rat-mite (p. 376, infra). 



5. The Protozoa parasitic in the blood of vertebrates are dis- 

 seminated by blood-sucking invertebrates, such as leeches, ticks, 

 or insects, which take up the parasites by sucking the blood of an 

 infected animal. Later on the parasite may be inoculated into a 

 second vertebrate host by the invertebrate when it sucks blood at 

 a later feed. In some cases the 'transference of the blood-parasite 

 may be effected in a purely direct and mechanical manner by the 

 invertebrate, but in most cases the invertebrate plays the part of a 

 true host, in which the parasite multiplies and goes through a cycle 

 of development. Hence in such cases also there is an alternation 

 of hosts and a complicated life-cycle, of which the life-history of 

 the malarial parasite is a good example (vide infra, p. 359). It 



