Sporozoa 271 



stages of growth, while certain others (coelozoic parasites) are found only 

 in body cavities. The merozoites, produced in merogony, may repeat the 

 cycle of growth and merogony but they eventually become differentiated 

 into gamonts (gametocytes). The production of gametes generally in- 

 volves schizogony (gamogo7iy, in this stage), although in well defined 

 anisogamy, the process is often limited to the production of microgametes. 

 The Telosporidea may be divided into three subclasses: Gregarinidia, 

 Coccidia, and Haemosporidia. In their various hosts, development of 

 most Gregarinidia is largely or completely extracellular; that of the other 

 two groups, mainly intracellular. Sporocysts are usually developed within 

 the oocyst of Coccidia but not in that of Gregarinidia. In the Haemo- 

 sporidia, the sporozoites are not enclosed within spore membranes. 



Subclass 1. Gregarinidia 



The gregarines are typically parasites of the digestive tract and 

 body cavities of invertebrates, although a few occur in tunicates and 

 Enteropneusta. The early development of many species occurs within 

 tissue cells, but the trophozoites usually emerge to complete the cycle in 

 some body cavity of the host. In other gregarines young trophozoites may 

 be attached to an epithelium but there is no intracellular stage. With the 

 exception of such genera as M er ogre gar ina and Spirocystis, in which intra- 

 cellular merogony occurs, older trophozoites are typically free in the 

 lumen of the digestive tract or in some other body cavity. In cephaline 

 gregarines (Suborder Cephalina), the transition from the attached stage 

 to the mature free trophozoite {sporadin, or sporont) often involves loss 

 of the epimerite, an organelle of attachment. This is to be expected 

 especially if the epimerite is firmly attached to the host's tissue or em- 

 bedded in a tissue cell. 



Gregarines vary widely in size, mature trophozoites ranging from about 

 10[jL to 3-4 mm in different species. Form of the body also varies con- 

 siderably (Fig. 6. 1). The fully grown gregarine is commonly an elongated, 

 often spindle-shaped organism, but there are a number of exceptions. 

 Among the elongated types, the body may be more or less cylindrical, or 

 it may be distinctly flattened. The typical individual gregarine also is 

 capable of undergoing contortions, which in some species, resemble 

 euglenoid movement of certain Euglenida. 



There is usually a well differentiated cortex which is sometimes rather 

 thick — e.g., 5-6[jL in Rhynchocystis porrecta (133) — and is composed of 

 two layers, the sarcocyte and the myocyte (Fig. 6, 1, H). The outer sur- 

 face is covered with a cuticle (epicyte), oflen marked with ridges or other 

 decorations (Fig. 6. 1, G). In Rhyjichocystis pilosa (133), cuticular "hairs" 

 (Fig. 6. 11, G) are attached to the ridges. The sarcocyte, the layer under- 

 lying the cuticle, is usually homogeneous in appearance. The myocyte 

 contains the myonemes characteristic of many gregarines. In some species 



