These will be described in more detail, as they are common and sometimes cause disease. A mature Sporozoon is generally covered outwardly with a delicate and firm layer or cuticle, which prevents the protrusion of pseudopodia, but may not prevent all movement. Contractile and other vacuoles do not occur in Sporozoa, because the method of nutrition is different from that of other Protozoa. The Sporozoa live as parasites' within the bodies of multicellular animals ; although they may often be found lying free in the various cavities of the body, they always pass some part of their life-history within a cell of their host, by the juices of which they are nourished.
A large Sporozoon named Monocystis may be obtained from the organs known as the seminal vesicles of the earth-worm. It seems impossible to find an earth-worm which is not highly infected with this parasite. In India the degree of infection is much greater than in more temperate climates. If a small portion of one of the seminal vesicles of an earth-worm be pressed into a drop of salt solution upon a slide the drop will be rendered turbid. With the naked eye the drop may be seen to contain minute white specks, which are the Monocysfcis. The drop should be Covered and examined under the microscope. The many parasites which are visible are not all alike, for they have reached different stages in their careers. The stages have received different names.
First Stage: Tlie Trophozoite.
A trophozoite feeds and grows, and stores up reserve food material ; many other animals and plants, besides nourishing themselves for the moment, store up food material for future use.
The body of the trophozoite is a single elongated cell surrounded by a thin cuticle, beneath which is a thin layer of clear ectoplasm. The endoplasm forms the mass of .the body; it contains many oval granules of reserve nutriment. The ectoplasm contains a layer of fibrils which, like the muscle cells of multicellular animals, are capable of contraction, and perform such movements as the animal exhibits ; these fibrils cannot easily be seen. The nucleus is like a vacuole, in which lies an opaque body called the karyosome. When the trophozoite is young, it lies embedded in the protoplasm of a special cell of the earth-worm which is called a sper
matoblast ; but as it grows it absorbs more and more of the spermatoblast until it becomes too large to be con tained within that cell. The mature trophozoite is found free in the cavity of the seminal vesicle.
Second Stage : The Gametocyte.
It has been mentioned that most organisms reproduce their kind by throwing off minute portions of their body, which grow and develop until they resemble the parent. Such portions always contain a nucleus, and are called germ cells, or gametes. Gametes are of two kinds, male and female, which are usually, but not always, produced by separate individuals. The two kinds of gametes are usually unlike one another in outward appearance, in which case the female gamete is called an ovum, and the male gamete a spermatozoon ; contrary to the rule, the gametes of Monocystis are all alike in outward appearance. Whether the two kinds of gametes be outwardly alike or unlike, their behaviour is the same, for they seek one another in pairs, and unite to form a single nucleated cell, which is called a zygote. The single nucleus of the zygote is formed by the fusion of two nuclei, one from each gamete. The formation of a zygote is the first stage in the reproduction of all organisms (except those few which reproduce their kind in the manner called parthenogenetic).
The trophozoite of Monocystis when full grown begins to form gametes, and is therefore referred to as a gametocyte. Just before the process commences, the trophozoites become associated in pairs : they lie side by side and surround themselves with a thin layer of transparent material. They are thus associated for purposes of reproduction within a spherical chamber which is called a cygt. The nucleus of each divides by mitosis. First into 2, then into 4, 8, 16, etc., until a large number of nuclei are formed. The process of division is not easy to see : it proceeds within the endoplasm, and is obscured by the granules. The result of the process however, can be seen without 14 AN INTRODUCTION TO BIOLOGY difficulty, for as soon as the many small nuclei are formed, they move towards the surface of the individual gametocyte which produced them, and lie each within a small portion of clear protoplasm. A gametocyte there fore becomes entirely surrounded by small units of protoplasm, each of which contains a nucleus. These units are the gametes. Changes of this kind occur similarly and simultaneously in both of the associated gainetocytes.
On their completion, the gametes become detached from their fixed position, and can be seen lying free in the cyst. They then become united in couples, and it is justly assumed that those formed by one gametocyte seek and unite with those formed by the other , the fact, however, is difficult to observe, because the gametes produced by each individual are exactly alike in appearance. As a result of their union a number of bodies called zygotes are formed. Each zygote becomes surrounded by a transparent coat of material called chitin. In this condition the zygote is referred to as a spore, because it leaves the body of the worm and lies in the earth until it is swallowed by another worm. The contents of the spore are saved from the effects of heat and drought by its resistent coat. The spores are oval in shape, with pointed ends like the outline of a boat.
The contents of a spore become divided into eight minute elongated bodies which are called sporozoites. The formation of the coat and the division of the contents of the spores takes place within the cyst. Cysts containing ripe spores and others containing ripening spores can be found without difficulty. Those cysts which contain ripening spores also contain opaque granular material, the remnant of the endoplasm of the parent gametocytes which is expended in nourishing the spores and in providing them with their hard coats.
Third Stage : The Sporozoite. There are eight sporozoites in every spore, each with a nucleus. When a spore is swallowed by a worm the sporozoites emerge and find their way into a special cell of the worm. They grow and become trophozoites, i.e. the first stage of the cycle.