534 BIOLOGY OF THE PROTOZOA 



corpuscle or Mood cell if it is one of the Hemosporidia. The adult 

 forms of Gregarinida are invariably extracellular or lumen-dwelling 

 parasites, young, growing stages alone being intracellular. Adult 

 forms of Coccidiomorpha are persistent intracellular parasites 

 throughout young, adult and reproductive phases. Although some 

 exceptional cases occur in both groups, these are essential differ- 

 ences between the two sub-classes Gregarinida and Coccidiomorpha. 

 All are typically uninucleate in the adult phase. 



Reproduction occurs either by agamogony or gamogony, the 

 latter involving fertilization. In one order of the Gregarinida, the 

 • Eugregarinida, the sporozoite grows directly into a gamont and 

 asexual reproduction is unknown. In a second order, the Schizo- 

 gregarinida, agamogony occurs either by equal division, internal 

 budding, or by multiple division. In Coccidiomorpha alternation 

 of generations is the rule and change of hosts is frequent. Multiple 

 division is practically universal. 



In both sub-classes the zygote undergoes metagamic divisions. 

 In Gregarinida and in Hemosporidia amongst the Coccidiomorpha, 

 the sporozoites are formed directly by divisions of the zygote; in 

 Coccidia the zygote divides into sporoblasts or sporozoite-forming 

 cells. In all cases except in Hemosporidia the sporozoites formed 

 in each such sporoblast are enclosed in a special capsule— Sporocyst 

 — by which the young organisms are protected against external 

 conditions. Hemosporidia are obligatory parasites in one host or 

 other throughout the entire life cycle otherwise they perish. 



Sub-class I. GREGARININA. 



The gregarines are typically celozoic or lumen-dwelling parasites 

 of the invertebrates, particularly of annelids and arthropods. They 

 vary in size from 10 /x to 16 mm. (Porospora gigantea) and are prone 

 to collect in masses in the intestine, a gregarious habit from which 

 the name of the group is derived. Saprozoic or osmotic in nutri- 

 tion they apparently do very little if any damage to the host organ- 

 ism, differing in this respect from the intracellular Coccidiomorpha. 

 The most frequent site of parasitism is the digestive tract and the 

 glands opening into it (e. g., Malpighian tubules) but the sporozoites 

 of some forms penetrate the wall of the gut and enter the body 

 cavity where they form cysts on the celomic side of the intestinal 

 wall or develop as free forms in the lumen of the seminal vesicles 

 (Monocystidae) or of other parts of the body cavity. 



Gregarines are widely varied in form as well as in size but so far 

 as the present accounts go they are similar in their protoplasmic 

 make-up. A peripheral outer layer of lifeless material forms the 

 epicyte which is equivalent to the pellicle or periplast of other 

 Protozoa. This is secreted by the ectoplasm and is frequently 



