THE president's ADDRESS, . 11 



animal that has devoured their former host, and pass unaltered 

 through its intestine, to be finally cast out with the dejecta. This 

 is almost certainly the method by which the common Monocystis of 

 the earthworm infects its host. The parasite produces resistant 

 spores in the worm; the worm is eaten by a bird, mole, frog, or 

 some other animal, through the digestive tract of which the 

 spores of the Monoci/stis pass unaltered ; 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 becomes again 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 be of frequent occurrence 

 amongst certain parasitic worms, such as Cestodes (tapeworms), 

 is j^robably also frequent amongst Protozoa ; but at present only 

 one case of it is known with certainty, that of the sjDecies of the 

 genus Aggregata, parasites of crabs and of Cephalopods, such as 

 the cuttle fish and the octopus. Tn 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 multi- 

 plies in its new host. If, as frequently happens, the crab is 

 devoured by a cephalopod, the parasite completes its life-cycle by 

 becoming once more a parasite of the cephalopod. 



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

 seminated in all cases, apparently, by bloodsucking 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 blood- 

 sucking invertebrate when it sucks blood at a later feed. In 

 some cases the transference of the blood-parasite from one verte- 

 brate host to another may be effected in a purely mechanical 

 manner by the invertebrate, but in most cases the invertebrate 

 is a true host in which the parasite multiplies and goes through 

 a cycle of development. Hence in such cases also there is an 



