THE SPOROZOA 



189 



and each such cluster of sporozoites resembles to a certain extent 

 an ordinary Gregarine spore, but has no enveloping membrane or 

 sporocyst (Fig. 41). Aggregata and Porospora are grouped together 

 as a tribe Gymnosporea in distinction to the ordinary Gregarines, 

 the Angiosporea, in which a chlamydospore is formed. The proto- 

 plasm of the sporonts may, in some cases, be entirely used up to 

 form sporoblasts, in which the sporulation is said to be complete, 

 but more often it is incomplete, with a more or less considerable 

 mass of residual protoplasm. 



The typical Gregarine spore contains eight sporozoites, and is 

 therefore said to be octozoic, but exceptionally only four sporozoites 

 are formed, as in Selenidium, hence tetrazoic. The sporozoites are 

 grouped in very various ways round a granular mass of residual 

 protoplasm, which contains the last remnants of the reserve 

 nutrition stored up by the sporont. The protoplasm of the 

 sporozoites is clear and finely granulated. Each sporozoite 

 is typically sickle-shaped with the nucleus in the middle. Some- 

 times the nucleus is at one extremity, and the sporozoite then 

 has a form more resembling a tadpole (Fig. 38). 



The spore -envelope or sporocyst consists of two layers, an 

 outer clear and delicate epispore, and an inner refringent and tough 

 endospore. Sometimes these two 

 layers are quite separate, or, on the 

 other hand, they may be intimately 

 united. In external characters the 

 spores show the greatest possible 

 variety of form and pattern, and 

 are frequently ornamented with 

 long tails or processes, which may 

 vary considerably even in closely 

 allied species, as in the species of 

 Cystobia infesting Holothurians (Fig. 

 38). Another remarkable feature 

 seen in some genera is the union of 

 the spores by their sporocysts to 

 form strings or ropes ("spores en 

 chapelet") (Fig. 34,/). 



With regard to the dissemination 

 of the spores, and the manner 

 in which they infect new hosts, 

 there is nothing to add to what 

 has been stated above with re- 

 gard to Monocystis (p. 163) and 



Sporozoa generally (p. 166). In no case is a true intermediate 

 host known to occur. The life-cycle of Gregarines is, in the vast 

 majority of cases, monogenetic, and consists of sporogony only, 



FIG. 33. 



Spores of Pyxiniarubecvla, Hamm. (par. 

 Dermestes spp.). a, a ripe spore showing 

 distinct epispore and endospore. 6, the 

 endospore set free after bursting of the 

 epispore. After extrusion of two polar 

 spheres (n), the sporozoites (s) escape from 

 the spore. (From Wasielewski, after Leger.) 



