Gregarina , the best known genus of the gregarinidoe, a division of protozoa, with no mouth and without the power of sending out the delicate filaments of sarcode characteristic of the foraminifera. They are among the lowest of the protozoa, parasitic, varying in size from a pin's head to half an inch in length; they infest the intestines of various animals, principally articulates, as the earth worm, lobster, beetle, and cockroach. They appear like a single cell, filled with a granular and fatty matter, with a nucleus and nucleolus; the external covering may be smooth, bristly, or ciliated. They have no definite organs, and the processes of nutrition and waste must be effected by the general surface of the body, as is common with internal parasites. In reproduction, the nucleus disappears, and the granular sarcode breaks up into little masses, which afterward become pointed, forming the so-called navicellae; these escape from the ruptured cyst, giving rise to active sarcode masses, which have the property of throwing out processes, like the amoeba; these in a suitable locality become developed into gre-garinae. One of the largest, said to be, with the exception of the yolk of the eggs of birds and some other animals, the largest known cell, the G. gigantea, is found in the intestine of the lobster; it is nearly two thirds of an inch long, and almost as wide.