Protozoa from Fishes. 391 



of the same parasite. It seems almost inconceivable that 

 there is a different invertebrate host to each genus of fish, 

 and hence it is inconceivable that each genus should be 

 infected with its own particular type of parasite. Variations 

 may be due to the age of the organism, or, what is much 

 more probable, to the different environment in which the 

 organism lives. Until more is known about the life-histories 

 of these forms, it seems superfluous to keep on creating new 

 species, and thus adding to the confusion already existing. 



The present work was carried on in the Dove Marine 

 Laboratory, (Jullereoats, and I wish to thank Professor Meek 

 and the staff of the laboratory lor their kindly help and 

 advice. 



I also wish to thank my sister and Mr. P. Gibson for 

 executing the diagrams illustrating this paper. 



Sept. 1915. 



Bibliography. 



Beneden, Van. ' Recherches sur les Bdellodes ou Ilirudinees ' (Brux- 



elles), 1863, p. 37. 

 Bentham, T. (1914.) " Some Observations on a Leucocytozoon of tbe 



Siskin ( Carduelis spinus)," Proc. Univ. Durham Phil. Soc. v. 



p. 139. 

 . "Note on a Hsemogregarine of JRaia batis." Report North- 

 umberland Sea-Fisheries Committee, 1914-15. 

 Brcmpt et Lebailly. " Description de quelquea nouvelles especes de 



Trypanosomes et d'Heinogregarines de Tel<5ost6ens marins." 



Coinpt.-rend. Acad. d. Sc. Paris, 1904, tome cxxxix. p. 613. 

 Johnstone, J. " Internal Parasites and Diseased Conditions of Fishes."' 



Rep. 1904 Lanes. Sea-Fisheries Lab. Univ. Liverp. p. 107. Ibid. 



1905, p. 152. Ibid. 1906, p. 192. 

 Minchin, E. A. ' An Introduction to the Study of the Protozoa,' 



pp. 371-378. 

 Minchin, E. A., & Woodcock, H. M. (1910.) "Blood-Parasites of 



Fishes occurring at Rovigno." Q. J. M. S. lv. p. 113. 

 Porter, A. (1U08.) "Leucocytozoon musculi.'' P. Z. S. 1908, p. 703. 

 Robertson, M. (1910.) " Life-cycle of Hcemogregarina nicorice." 



Q.J. M. S. lv. p. 741. 

 Saville-Kent, W. ' A Manual of the Infusoria,' vol. ii. p. 273. 



EXPLANATION OF THE PLATES. 

 Tlate XVI. 



JFig.l. Hcemogregarina cotti. 1 = curved form in blood-corpuscles of 

 Cottus. 2 a & #& = macrogametocytes from gut of Callio- 

 bdella. <3a = forin from spleen of Cottus. <3&=rnicrogameto- 

 cyte. ^=macrogametocyte with discbarge of karyosomes. 

 5 « & 5 6 = stages in schizogony. 



Fig. 2. Dorsal blood-vessel of Ichthyobdella anarrhichce, showing spores 



