Gregarinida

Gregarinida 

an order of class Sporozoa, phylum Protozoa. Parasites of the intestines and body cavities of invertebrate animals (predominantly annelid worms and insects) and, among the chordates, of tunicates.

The body of the gregarine consists either of a nonseg-mented cell or of a cell divided into two or three sections. The posterior section of the body, the deutomerite, contains the nucleus. Anterior to it is the protomerite. on which there may be an ectoplasmic process called the epimerite, which serves to attach the gregarine to the tissues of its host. The body of the gregarine is covered with a thick plicate membrane (pellicle), under which, in the ectoplasm, there are various fibrillar structures. The endoplasm is granular and rich in glycogen. Gregarines are motile; the mechanism of movement is apparently not reactive, as was thought previously, but is rather the result of the contraction of myonemes (fibrils that lie within the ectoplasm). Order Gregarinida is divided into two suborders. Representatives of the small suborder Schizogregarinida are characterized by the presence in their life cycle of asexual reproduction (schizogony). The great mass of gregarines belongs to the suborder Eu-gregarinida, which is characterized by the absence of schizogony. The life cycle of Eugregarinida proceeds according to the following pattern: Upon entering the phase of sexual reproduction, the adult gregarines (called gamonts) unite in pairs, forming a syzygy surrounded by a cyst. The nucleus of each gamont divides repeatedly and the cytoplasm separates around each of the daughter nuclei, forming the gametes; pairs of these gametes then join (copulate) to form zygotes. Each zygote is covered by a membrane and is transformed into an oocyst. Inside the oocysts eight small cells, or sporozoites, are formed which, when they encounter a suitable host, may become the source of new infection. After they attain adulthood the cycle is closed. All gregarines are haplonts with zygotic reduction; the zygote alone is diploid. Meiosis and reduction of the number of chromosomes occurs in the oocyst during the first division of the zygote.

REFERENCES

Zhizn’ zhivotnykh, vol. 1. Moscow, 1968.
Traité de zoologie, vol. 1. Published by P. P. Grasse. Paris, 1948.

O. I. CHIBISOVA