STRUCTURAL MODIFICATIONS OF PROTOZOAN PARASITES 175 



expected that some of them, at least, would have acquired the parasitic 

 mode of life. The enormous literature which annually appears in 

 connection with the protozoan parasites, perhaps better than anything 

 else, shows that such an expectation is well founded (Liihe ('06) 

 points out that in connection with the blood-dwelling protozoan 

 parasites alone there are from 600 to 700 papers published every year), 

 and every division of the protozoa numbers among its genera some 

 that are wholly or in part parasitic. 



I. STRUCTURAL MODIFICATIONS AND MODE OF LIFE OF 

 PROTOZOAN PARASITES. 



It is not a too sweeping generalization to state that every living 

 thing, large enough to contain another living thing, is subject to inva- 

 sion by parasites. The protozoa, themselves single cells, often play 

 the part of host to smaller protozoan cells, and parasites often infect 

 even the nucleus of ameba, paramecium, vorticella, and other types. 



If the imagination were allowed full play, it would not be very dif- 

 ficult to work out a logical hypothesis as to the transition of different 

 kinds of protozoa, from a free life in ponds and ditches to a parasitic 

 life in the digestive tract or other organs of various animals. It is 

 certainly true that representatives of all groups of protozoa have from 

 time to time in the past become adapted to life within some other 

 animal or plant, and it is equally true that in many cases their presence 

 is harmful to the host and may become fatal. Frequently such para- 

 sites have become so modified by their changed mode of life that their 

 structures furnish little or no hint as to the original or primary form. 

 Such is the case in the majority of sporozoa, where every member is a 

 parasite, the origin of the group, as a whole, whether from rhizopods 

 or flagellates, being purely conjectural. In some cases the method of 

 locomotion by pseudopodia formation, the presence of a contractile 

 yacuole, and the mode of reproduction indicate rhizopod affinities; 

 in other cases the evidence of degenerating structures, taking 

 place before our eyes, as it were, at the present time, is unmis- 

 takable, and such forms write their own phylogenetic history. 

 This is true of some members of the blood-dwelling parasites, 

 where, as in Herpeiomonas (Leishmania) donovani, the adult 

 organism is a flagellated protozoon in the gut of its definitive host 

 (bugs of the genus Cimex), but becomes an intra-cellular parasite 

 without motile organs of any kind in the intermediate host man; 

 or in Trypanosoma noctuce (H emoproteus noctuce), where a highly 

 differentiated free-swimming flagellate becomes an intra-cellular 

 blood parasite of the bird (Glaucidium (Athene} noctuce), and with 

 a much simpler structure (see page 244). From such evidence 



