618 THE AMERICAN NATURALIST. | [Vor. XXXIX. 
intestinal epithelium of the mosquito and comes to rest in the 
periintestinal tissue. There is first a period of growth, during 
which the element assumes a spherical form and becomes 
inclosed by a delicate wall. No actual cyst is developed. 
Eventually, the sporozoites are produced directly from the 
protoplasmic mass, the spore stage being omitted.  Excepting 
for the fact that fertilization is postponed until after the removal 
of the parasite from the vertebrate host, that part of the Acysto- 
sporean lite history which 1s passed in the mosquito corresponds 
exactly to that part of the Coccidian life history which is passed 
in the cyst. 
The Acystosporea are parasitic in the blood of mammals and 
birds. Intermediate hosts are known in many cases, and they 
are.generally assumed in the rest. The Haemosporea, on the 
other hand, infest reptiles and batrachians. Intermediate hosts 
are not known,! nor can we readily believe that such forms are 
much preyed upon by Diptera. However greatly we may dread 
the mandibles of the mosquito, the Chelonia are probably indif- 
ferent to its attacks. Moreover, in the case of Lankesterella of 
the frog, it has been shown by Hintze ? that sexual reproduction 
takes place in the blood, and that the motile zygotes so formed 
gain the epithelium of the intestine. Here they encyst, the 
cysts so formed eventually reaching the exterior by being 
carried out with the fzeces of the host. 
If the life history of Lankesterella may be taken as represent- 
ative of that of the Hzemosporea, we find that the two sub- 
orders of the Haemosporidia differ only in that the Acystosporea 
have acquired an intermediate host. While this is indicative of 
more extreme specialization, it is not difficult to imagine how it 
may have been brought about. The warm-blooded vertebrates 
have doubtless long been preyed upon by blood-sucking arthro- 
pods. The crucial point in the bringing about of this change of 
life on the part of the parasites was that some such blood- 
sucker would be unable to digest them. This would be merely 
a matter of chance. There was probably one time in their 
! A case has, however, recently been described. 
* Zool. Jahrb., AMA. f. Anat., vol. 15, 1902, pt. 4, p. 693. 
