240 THE SPOROZOA 



who gave them as a class the name Haemosporidia or Haemocytozoa, and 

 by other investigators. In 1894 Labbe" brought forward detailed and 

 extended researches upon these parasites, and described many new forma. 

 The concluding years of the nineteenth century have brought a very rapid 

 increase in our knowledge of the malarial parasites, and the labours of 

 Ross, Qrassi, and many others have revealed their complete life-history, a 

 chapter of biology of the greatest practical importance as well as of scientific 

 interest. At present it is amongst the Haemosporidia of cold-blooded 

 Vertebrata that researches are most needed. 



(a) Occurrence and Habitat. The Haemosporidia are found com- 

 monly as blood -parasites in mammals, in birds, and in all the 

 existing orders of reptiles, except perhaps the Rhynchocephala. 

 Amongst amphibia they appear to occur abundantly in the frog, 

 at least, which has been credited with harbouring no less than five 

 species, distributed amongst four genera, of these parasites ; but it 

 is highly probable that improved knowledge will bring about 

 reductions in this list. From Urodela, on the other hand, only 

 doubtful species have been recorded. In fishes also Haemosporidia 

 were generally considered to be conspicuous by their absence, but 

 very recently Laveran and Mesnil [79, 79a] have described species 

 infesting rays, soles, and blennies respectively. If we except the 

 cases where a part of the life-cycle is passed through in an inter- 

 mediate host, there is no record of their occurrence in Invertebrates, 

 with the exception of one very doubtful species (Haemogregarina 

 nasuta, Eisen) stated to occur in the walls of the blood-vessels 

 and the mesentery of an Annelid (Edipidrttus frigidus). 



The principal habitat of Haemosporidia is the red blood- 

 corpuscles of their hosts, but they may be found also in the 

 leucocytes, and in the cells of certain organs, especially the 

 spleen and bone -marrow. It is not uncommon for the repro- 

 ductive phases of the parasite in the vertebrate body to be rare 

 or absent in the blood of the peripheral circulation, in which only 

 growing trophozoites or gametocytes are to be found, while the 

 " rosettes " and other stages of schizogony occur only in the more 

 slowly flowing blood of the brain, liver, kidney, and other viscera. 

 The situation of the endoglobular parasite is always within the blood- 

 corpuscle or cell it attacks, 1 and not, as supposed originally by 

 Laveran, merely one of attachment to the corpuscles. In the case 

 of the forms parasitic upon Vertebrata other than mammals, the 

 nucleus of the haematid is often displaced by the parasite, proving 

 clearly its internal position. Occasionally the nucleus itself may 

 be attacked ; a good example of this is seen in the form parasitic 



. * The view of Laveran that the parasites are attached ("accoles ") to the cor- 

 puscle has recently been revived and supported by Argutinsky [61, 1901], but his 

 statements have been criticised and contradicted by Schaudinn [94a], who is strongly 

 in favour of the view put forward above. 



