Sporozoa 279 



involved. Depending upon the species, trophozoites (Fig. 6. 10, F-H) 

 grow to maturity in the intestine of a crab or a lobster. At the end of 

 the growth period, each gregarine adheres to the lining of the rectum and 

 undergoes encystment as an individual, not as a member of a syzygous 

 pair. Encystment is followed by a series of rapid nuclear divisions and 

 then the production of a number of small spherical gymnospores (Fig. 

 6. 10, A), each of which is composed of radially arranged "merozoites." 

 When the cyst membrane finally ruptures, the gymnospores are released 

 into the sea water. Later stages develop in a molluscan host. After reach- 

 ing the mantle cavity of a lamellibranch, gymnospores may penetrate the 

 epithelium of a gill-filament or of the foot. In addition, phagocytes may 

 ingest gymnospores at the epithelial surface and take them into the 

 tissues. After passing through the epithelium, the gymnospores develop 

 to maturity in the tissue spaces. According to Hatt (42), the most plau- 

 sible interpretation of his observations is that the "merozoites" of the 

 gymnospores become differentiated into two types of gametes which un- 

 dergo anisogamy. Each zygote apparently develops directly into a young 

 sporozoite. The mature sporozoite of Nematopsis (Fig. 6. 10, D, E) is 

 enclosed in a spore membrane surrounded by a gelatinous sheath, whereas 

 the sporozoites of Porospora remain naked. In either case, a crustacean 

 host apparently becomes infected by eating infected molluscan tissue. 



In terms of the usual cycle, it might be assumed that transfer from the 

 crustacean to the molluscan host interrupts the normal sequence of 

 gamogony and syngamy. Such an interpretation would imply that gymno- 

 spores contain immature gametes which mature in the molluscan host. 

 However, gymnospores are produced by single encysted gregarines, not 

 by pairs in syzygy, and two types of gametes seem to develop within each 

 gymnospore. Consequently, the mechanism of sexual differentiation in 

 the Porosporidae remains an interesting problem. 



Taxonomy 



The Gregarinidia are divided into two orders on the basis of a major 

 difference in life-cycles. In the Schizogregarinida merogony occurs in the 

 asexual phase; in the Eugregarinida, there is no merogony. 



Order 1. Schizogregarinida. The trophozoite undergoes nuclear divi- 

 sion at maturity to produce a multinucleate schizont. Merogony then 

 occurs and the surviving merozoites repeat the cycle of growth and merog- 

 ony, perhaps several times before the merozoites develop into gamonts. 

 Merogony may occur either inside a tissue cell, or in a body cavity. Extra- 

 cellular types may be attached to an epithelium throughout much of the 

 growth period. Gametocysts and spores are developed as in gregarines 

 generally. The Schizogregarinida are parasites of various polychaetes, 

 gephyrean worms, insects, and (rarely) tunicates and Enteropneusta. 



The order has been divided into two families (80), Ophryocystidae and 



