Sporozoa 317 



Three genera are included: Myxidium Biitschli (Fig. 6. 28, E), Sphaeroniyxa 

 Thelohan (Fig. 6. 28, B), and Zschokkella Auerbach (Fig. 6. 28, P). 



Family 3. Myxobolidae. The spores contain two polar capsules at one 

 end and an iodinophilous vacuole. 



The family includes the following genera: Henneguya Thelohan (Fig. 6. 28, N), 

 Myxobilatiis Davis (25; Fig. 6. 26, F), Myxobolus Thelohan (Fig. 6. 26, A, B), Thelo- 

 hanellus Kudo (Fig. 6. 28, Q, R), and Unicattda Davis (25). 



Family 4. Myxosomatidae. The spores contain two or four polar cap- 

 sules and no iodinophilous vacuole. There are only two genera: Agarella 

 Dunkerly (Fig. 6. 28, S) and Myxosoma Thelohan (Fig. 6. 28, L). 



Order 2. Actinomyxida. This group includes organisms whose discov- 

 erer, Stole, believed that they should be considered Mesozoa rather than 

 Protozoa, in view of their complexity. The pansporoblast, or pansporo- 

 cyst, typically develops eight spores, each with a membrane composed of 

 three valves (Fig. 6. 26, E). In some species, each valve is drawn out into 

 a horn, or spine, which may or may not be bifurcated. There are also 

 three polar capsules, but the number of sporoplasms ranges from one to 

 a hundred or more in different species. Species are known from sipuncu- 

 lids and tubificid annelids. 



The life-cycle of Triactinomyxon legeri (84) is probably representative. 

 The mature spore (Fig. 6. 30, D) contains a sporoplasmic mass in which 

 lie 24 uninucleate sporoplasms and three or more residual somatic nuclei. 

 The sporoplasms eventually fuse in pairs to produce 12 binucleate stages. 

 Subsequently, the mass breaks up into several fragments which leave the 

 spore membrane separately (Fig. 6. 29, A, B) and liberate the binucleate 

 stages as small amoebae. Eachamoebula (Fig. 6. 29, C) grows for a time 

 and then undergoes nuclear division. Two of the nuclei, with associated 

 cytoplasm, become peripheral in position and, as the cystogenous cells, 

 produce a cyst around the remaining protoplasm (Fig. 6. 29, D). This 

 stage is called a jDansporoblast, or after the membrane is completed, a 

 pansporocyst (84). The central binucleate mass next divides into two cells, 

 one of which reproduces more rapidly than the other, so that a number of 

 small cells and a few larger ones are produced (Fig. 6. 29, E, F). Ani- 

 sogamy involves fusion of a large cell with a small one. 



In development of sporoblasts within the pansporocyst, the nucleus of 

 each zygote gives rise to seven nuclei (Fig. 6. 29, G). Three of these, with 

 associated cytoplasm, migrate to one end of the developing spore as cap- 

 sulogenous cells which later produce polar capsules (Fig. 6. 29, H). Three 

 other cells, which are to produce the valves of the spore membrane, mi- 

 grate to the opposite pole of the sporoblast (Fig. 6. 29, I, J). The seventh 

 nucleus initiates a series of mitoses resulting in 27 nuclei. Three of these 

 are residual somatic nuclei; the other 24 become sporoplasmic nuclei. 



