PROTOZOA 83 



The vegetative stage {trophozoite) has usually a definite shape, but 

 in some haemosporidia is amoeboid. Its fission (agamogony), if such 

 occur, is multiple, and is usually known as schizogony^ the term schizo- 

 zoites or merozoites being applied to the offspring. Its single nucleus only 

 divides to form those of the young into which this stage breaks up, 

 but owing to such division the body may be for a while multinucleate. 

 The trophozoite of one of the two orders (the Coccidiomorpha) 

 remains intracellular: in the other order (the Gregarinidea) it after 

 a time outgrows its cell host. Save in one suborder (Eugregarinaria), 

 it passes through the usual phase of agamogony before giving rise to 

 gamonts, but in the Eugregarinaria agamogony is omitted, and the 

 members of the single vegetative generation become gamonts, which 

 provide for the increase of the species by the formation of many 

 gametes in both sexes. The gamonts may be free or intracellular. 

 Free individuals are often able to adhere by a sticky secretion, form- 

 ing what is known as a syzygy. When gamonts so adhere (Figs. 76, 

 6; 77 B) they do so in pairs^ whose members are to be the parents 

 of gametes that will unite reciprocally. Syngamy is isogamous in a 

 few of the Gregarinidea, but is usually anisogamous, and in the Cocci- 

 diomorpha becomes an oogamy (p. 26). In some cases, perhaps in 

 all, the first division of the zygote is a reduction division, so that 

 nearly the whole of the cycle is haploid. 



Order COCCIDIOMORPHA 



Telosporidia in which the adult trophozoite remains intracellular; 

 and the female gamete is a hologamete. 



Typically the members of this order are parasites of the gut, but 

 more than once they have come to infest the blood. One such invasion 

 gave rise to the suborder Haemosporidia. The rest of the group con- 

 stitute the Coccidia. 



Suborder COCCIDIA 



Coccidiomorpha, for the most part gut parasites ; of which the zygote 

 is non-locomotory ; the sporozoites are nearly always encased ; and 

 the gamonts often form a syzygy. 



Eitneria (Fig. 74) is parasitic in the intestinal epithelium of 

 various vertebrates and invertebrates. E. schubergi, from the intestine 

 of the centipede Lithobius, may be described as a type of the suborder. 

 The spherical trophozoite (agamont) undergoes schizogony (agamo- 

 gony) by multiple fission within the epithelial cell which it inhabits. 

 The spindle-shaped schizozoites (agametes) being set free into the 

 cavity of the organ, each infects another cell in which it grows like 

 its parent. After some days of this there occur fissions in which the 

 ^ The term syzygy should perhaps be restricted to such pairs. 



6-2 



