Page:Popular Science Monthly Volume 59.djvu/200

190 Sporozoa is that, notwithstanding the many kinds and the wide distribution in all sorts of hosts, the life history of the parasites invariably conforms to the same type, a fact which has recently been used to good advantage in working out the development of the malaria organism.

Like all the unicellular animals, or Protozoa, the Sporozoa are minute bits of protoplasm provided with a membrane and a specialized spherical portion of the inner protoplasm called the nucleus. Unlike the other Protozoa, they are entirely devoid of motile organs and are, in consequence, quiescent. In classifying them, advantage has been taken of the different modes in which they form spores, or germs, by which they are reproduced. In some, known as the Telosporidia, all the protoplasm of the parasite is used to form the spores, and the parent cell dies or disappears with each sporulation, which thus represents the end of the individual parasite. The individuals of the second group, known as the Neosporidia, form spores, without using all the protoplasm, and continue to live after each sporulation. This group comprises the less-known forms of Sporozoa, and is of considerable economic importance as the cause of epidemics among silkworms, brook trout and other fish, etc.

The Telosporidia are further divided according to the mode of life. Some of them, known as the Gregarinida, live in cavities of the body of many forms of invertebrates, but rarely in vertebrates; others, the Coccidia, live in epithelial cells lining the cavities of both invertebrate and vertebrate hosts. It may be remarked, parenthetically, that the cause of cancerous growths in man is claimed by many to be organisms belonging to this group of Sporozoa. The question remains in considerable doubt, however, and, despite the great mass of literature, no positive results have appeared. The last group finally of the Telosporidia is the Hæmosporidia, comprising parasites which, like the malaria-organism—Plasmodium malariæ—live in blood corpuscles of vertebrates.

All these different types of Telosporidia begin life as minute germs called sporozoites, which make their way into the new host through the intestine, being taken in with the food. The life history, after this ingestion, follows slightly modified paths in the different types, and, for purposes of comparison, I will describe these processes in the gregarine, the coccidium and in the hæmospore Plasmodium malariæ, thus representing each of the subdivisions of the Telosporidia.

The sea-squirt, or Tunicate, Ciona intestinalis, is the host of a gregarine Monocystis ascidiæ, which is so widely distributed that it is almost impossible to find a Ciona without them. The complet e life history of the parasite has been fully worked out by Prof. M. Siedlecki,