Protozoa, in which the organism is either devoid of all special organs of locomotion or is provided with slowly changeable pseudopodia; nutrition takes place solely by absorption; endoparasitic in all stages of life.

There is one class - the Sporozoa.

Class Sporozoa

Endoparasitic Protozoa, rarely motile, reproducing by spores which are usually provided with an envelope (chlamydospores), and are seldom naked. The spores give origin in turn to a variable number of germs, sporozoites or falciform bodies. Conjugation may take place, but as a rule is a mere juxtaposition of two or more individuals.

There are four sub-classes, the Gregarinida, including the Coccidiidae, the Amoebosporidia, Sarcosporidia, and Myxosporidia.

(I) Gregarinida

The Sporozoa contained in this sub-class are, with few exceptions, in their early stages intracellular parasites. The Cocci-' diidae are sometimes found in the connective tissues, but as a rule remain within the cell-host until encystation takes place. The other Gregarinida quit it at a certain period of growth and enter either the coelome or the cavity of the digestive tract.

The Coccidiidae are round or oval in shape, and in the adult condition motionless; they are mostly minute in size, but range from "025 up to 1 mm. in the Klossia of the Cephalopoda. The other Gregarinida are usually elongate, very rarely globular (Adelea), and are often flattened. An anterior extremity is distinguishable, and may be furnished with organs of adhesion. The majority are small, ooI-'O2mm. in length; some attain a fair size, e. g. Monocystis magna of the Earthworn 1/5 in. (5 mm.); but the giant of the group is Porospora gigantea of the Lobster, which may attain 3/4 in. The adult is sometimes quite inert, sometimes active, gliding evenly along, but the mode in which locomotion is effected has not been discovered. Changes of shapes have been observed, especially in the young form; they are apparently due to movements or local accumulations of protoplasm, which may or may not cause movements. The subclass is divisible into two orders, the Monocystidea, in which the body is simple, and the Polycystidea, in which it is divided by two septa into three segments, with the doubtful exception of Porospora and the certain one of Gamocystis, which has two segments, one large, the other very minute in size, corresponding probably to the two first segments of other forms.

The first segment is the epimerite; it is the part from which the other two segments bud out, and is in later stages an organ of fixation in the cell-host. It is either immersed in the protoplasm of the cell-host, or if cup-shaped, as in Lophorhynchus insignis, grasps the ends of a number of intestinal epithelial cells. The second segment is the protomerite, the third and by far the largest, the deuteromerite, in which the nucleus is lodged. A Polycystid possessing all three segments is known as a Cephalin or Cephalont. Sooner or later it loses the epimerite, more or less completely, and is then known as a Sporadin or Sporont1.

Klossia alone among Coccidiidae has a distinct cell-membrane. The remaining Gregarinida have a delicate cuticula, which is often finely striated in a longitudinal direction. The striae in Lophorhynchus insignis are due to the cuticle consisting of delicate vertical lamellae set on edge side by side. Delicate hair-like processes are present in Callyntrochlamys, lobular processes in Conorhynchus2. The retinacular processes of the epimerite, when present, are probably formed by the ectoplasm. The protoplasm of the body, except in Coccidiidae, is usually divisible into a clear exoplasm and a granular fluid endoplasm or entocyte. The former is best developed at the two extremities of the body and constitutes the chief bulk of the epimerite; an outer firmer layer, the sarcocyte, is very commonly differentiated from it. In the Polycystidea it is frequently traversed by fine fibrils, usually transverse, rarely reticular, homogeneous, or in Porospora, composed of granules apposed in linear series. To these fibrils a muscular function was at one time assigned, and they were supposed to constitute a special contractile layer or myocyte. But the Gregarines in which they best developed are the most inert. The septa of the Polycystidea are constituted by exoplasm or sarcocyte.

Movements in the endoplasm are never propagated through them. The granules of the endoplasm sometimes display molecular movements; they are variable in size and shape; and in some instances either contain or consist of an amyloid substance. Colouration is rare, and appears to be due to the intestinal secretions of the host. Contractile vacuoles are absent; non-contractile are seen in Conorhynchus Echiuri. The nucleus is single; it is large when full grown, vesicular, with chromatin globules or ribbons. It lies in the endoplasm, with the movements of which it shifts, and in the Polycystidea always in the deuteromerite, into which it migrates from the epimerite before the septa are complete.

1Schneider entertains the idea apparently that the epimerite is the equivalent of a monocystid Coccidiidan, the proto- and deutero-merite, the equivalent of its spores. See his Tablettes Zoolo-giques, p. 113 et seqq.

2 The species of Monocystis which inhabit the vesiculae seminales of the Earthworm live within the spermatospores. When the latter give origin to spermatozoa the Gregarine appears to be coated with cilia; and the moult that was supposed to take place merely signifies the escape of the parasite from the sperm-blastophore.

The Coccidiidae encyst as soon as they have attained their definitive size. The cyst has rarely a single, but usually a double wall, is very resistent, and has in Eimeria and Coccidium a micropyle. The contained protoplasm gives origin to a single spore (Monosporea), to two or four (Oligosporea), or to a large number (Polysporea). The spore is naked in Orthospora, Eimeria and Gymnospora, in other instances with a spore-membrane which may be double. The spore-protoplasm gives origin to a single elongate falciform body in Coccidium oviforme of the Rabbit, to 2 in some species of Klossia, to 2-4 in Cyclospora, to 4 in Orthospora, to many in Eimeria, etc. In the last-named genus they are formed on one aspect only of the spore. A small quantity of protoplasm, the 'nucleus de reliquat,' may remain after the formation of the falciform bodies. The division of the nucleus in the formation of spores and falciform bodies has been observed in some instances. The falciform body may move while still within the spore case, more energetically after its escape. It does so either by bending into a semicircle, by gliding, rotating round a point, or by feeble amoeboid motions (Eimeria falciformis). It probably enters a cell whilst in the falciform condition.