BLOOD -PARASITES OF FISHES OCCURRING AT ROVIGNO. 129 



There is another point which is of considei^able importance. 

 In all the above instances these long-, slightly curved forms 

 (adult merozoites) were readily observed free, as " vermicules," 

 in smears as well as in the living- condition. This is quite 

 natural if, as we consider, tliese vermicules are ready to 

 penetrate a fresh host-cell, probably in an internal org'an, and 

 there give rise by schizogony to the sexually differentiated 

 forms. We think it will be useful to distinguish these free 

 individuals as schizokinetes, meaning thereby a tem- 

 porarily motile schizont. 



In striking contrast to the above cases we have the entire 

 absence, so far as we are aware, of free individuals of either 

 of the large types of H. rovignensis, although the great 

 majority of them seem to be full-grown and mature. We have 

 no grounds whatever for thinking that these individuals 

 become free fx'om the corpuscle while still in the fish. 



This difference in behaviour also points to these large forms 

 of the Trigl a -parasite being gametocytes and not schizo- 

 kinetes. On the supposition, which is most probable, that 

 the sexual process of these haemogregarines takes place in a 

 leech, these is no reason why we should expect to find gameto- 

 cytes liberated in the blood of the fish, because they are await- 

 ing the stimulus of the invertebrate host before being set free 

 from the corpuscles. We may compare in this respect other 

 intra-cellular blood-parasites, for instance, Halteridium, 

 Proteose ma, or Leucocy tozoon of birds, with which we 

 have had much to do. If a drop of blood containing these 

 parasites is taken and smeared quickly, scarcely any of the 

 ripe gametocytes present have ruptured the host-cell and 

 become rounded off. It is only when the blood is allowed to 

 cool for a short time in the living condition that the sexual 

 forms become free, as indeed is well known. It may be said 

 that if the large forms of H. rovignensis are gametocytes, 

 we ought to have seen some of them at any rate become free 

 in the living preparations examined. This does not follow at 



view of the situation and also on account of the nuclear structure (cf . 

 also below, p. 149, footnote). 



VOL. 55, PART 1. NEW SERIES. 9 



