THE NEOSPORIDIA 423 



and from which an amoebula is liberated, as in other Neosporidia 

 (Amoebogenus). 



Order V. : Haplosporidia. The distinctive features of this order 

 are for the most part of negative character, and, as the name im- 

 plies, the tendency is towards simplicity in structure and develop- 

 ment. The spores are without the polar capsules which are so 

 marked a peculiarity in the four previous orders, and have the 

 form of simple cells, each with a single nucleus, and with or without 

 a sporocyst, which, however, when present, is not formed by distinct 

 parietal cells. 



In organisms of such simple structure, the absence of distinctive 

 peculiarities renders the limits of the group indefinite, and the 

 affinities of its members vague and undecided, and it is possible 

 that the order Haplosporidia, as generally understood, is a hetero- 

 geneous assemblage, many members of which present only develop- 

 mental analogies tp the true Neosporidia that is to say, a simi- 

 larity in the life-history which is an adaptation to a similar mode 

 of life, and not a true indication of genetic affinity. L6ger and 

 Duboscq (646) point out that the characters " peu limitatifs " 

 of the Haplosporidia would suit Protista of the most diverse affini- 

 ties, and scarcely mark them off from yeasts or Chytridinese. With 

 the exception of the family Haplosporidiidce, they regard the group 

 Haplosporidia as purely provisional, and comprising heterogeneous 

 forms with undecided affinities. 



The life-cycle of a typical Haplosporidian parasite is very simple. 

 The initial phase is an amcebula or planont, which multiplies by 

 fission, division of the nucleus being followed by division of the 

 body to form two planonts, which may continue to divide for many 

 generations. From a planont arises ultimately a plasmodial phase, 

 the result of divisions of the nucleus without corresponding divisions 

 of the body, which grows to a relatively large size. The plasmodium 

 is the principal trophic phase. It may multiply by plasmotomy or 

 by schizogony, or may proceed to spore-formation, and then it 

 divides into as many cells as there are nuclei. The cells formed in 

 this way are either sporoblasts, each of which becomes a single 

 spore (Oligosporulea), or they represent sj^>ronts (" pansporo- 

 blasts "), which give rise each to a cluster of spores (Polysporulea). 

 The spores are usually simple rounded bodies invested by a 

 more or less distinct protective membrane, which in rare 

 instances becomes a definite sporocyst prolonged even into tails 

 or spikes. 



The Haplosporidia were divided by Caullery and Mesnil (802) 

 into three families. In order to include forms more recently dis- 

 covered, Ridewood and Fantham have extended the classification, 

 and recognize two suborders : 



