470 



TELOSPORIDIA 



Reproduction by Schizogony or Sporogony, — The gregarines are essentially 

 parasites of the invertebrata, not being found in true vertebrates, though 

 known in Amphioxus and Ascidians. They are generally found in the Arthro- 

 poda and worms, and appear to be non-pathogenic. 



Their life-history rnay be briefly described as follows: — The young tropho- 

 zoite enters a cell, generally of the alimentary canal, in which it grows, appar- 

 ently causing considerable damage, for the cell first swells and afterwards 

 degenerates. 



In the meanwhile the parasite has found its way either wholly or partially 

 out of the cell, to the remains of which, however, it is attached until all 

 nourishment is extracted, when it becomes ccelozoic — i.e., it leaves the remains 

 of the cell, and dwells either in the alimentary canal, the ccelome, or the 

 blood stream. The young parasite is unicellular, having its cytoplasm divided 

 into ectoplasm and endoplasm. The ectoplasm is hyaline, and when fully 

 developed has three layers: (i) external epicyte, (2) middle sarcocyte, {3) in- 

 ternal myocyte. 



The last-named contains contractile fibrils called myonemes. The endo- 

 plasm is granular, and contains a well-developed vesicular nucleus. A uni- 

 cellular gregarine, such as this, belongs to the suborder Acephala, and is 

 illustrated by Monocystis agilis Stein, which is found in the vesiculae seminales 

 of LumbricMs terrestris. 



On the other hand, in such a gregarine as Pyxinia frenzeli Laveran and 

 Mesnil, a part of the sporozoite grows out from the cell, and into this external 

 portion the nucleus travels. The portion left in the cell is known as the 

 ' epimerite,' while the external portion, growing considerably, is divided by a 

 septum into a ' protomerite ' near the ceil, and a ' deutomerite ' away from the 

 cell. 



The parasite in this condition is called a cephalont, and belongs to the sub- 

 order Cephalina. 



In due course the trophozoite, whether entirely or partially in the cell, is set 

 free, either by bursting its way out, or by a separation between the epi- and 

 proto-merite. The cell now degenerates, and the parasite may be called a 

 sporont. Sporogony takes place by two sporonts or gametocytes lying side 

 by side becoming enclosed in a cyst with two walls — an external epicyst and 

 internal endocyst. 



The karyosomes of the nuclei of the gametocytes now break up into 

 chromidia, which collect to form the generative nucleus. This divides by 

 mitosis to form a large number of nuclei, which, travelling to the surface and 

 surrounding themselves with cytoplasm, become detached as gametes, the rest 

 of the cell forming a nucleus de reliquat or residual mass. Thus, inside the cyst- 

 walls there are gametes from two separate parasites and two residual masses. 



Gametes presumably from different individuals fuse and form a true zygote 

 with a synkaryon. Each zygote represents a sporoblast, and each sporoblast 

 becomes a single spore by the secretion of a chitinous cuticle and the contrac- 

 tion of its protoplasm (called by Minchin the ' sporoplasm ') . This sporoplasm 

 divides by amitosis into eight sporozoites and a residual mass. During this 

 process the two residual masses of the original cyst disappear. The cyst now 

 contains only the spores which used to be called pseudonavicellae. These 

 spores are intended to convey infection to a new host, in the intestine of which 

 they set free the sporozoites, which promptly attack either the cells of the 

 intestine or some other organ, thus completing the cycle of sporogony. 



Schizogony. — This is uncommon in the gregarines, being only found in the 

 Schizogregarinidse. It is typically seen in Schizocystis, in which the tropho- 

 zoite grows into a schizont, whose nucleus divides into a large number of 

 daughter nuclei, round each of which a portion of cytoplasm gathers, forming 

 a merozoite. This, escaping from the schizont, infects another cell, thus com- 

 pleting the cycle of schizogony. Some of the merozoites may develop by 

 sporogony. 



Classification. — The gregarines are classified as foUpws: 



