444 EVOLUTION OF ANIMALS Part V 



Class Sporozoa 



Characteristics. The sporozoans are without exception the parasites with 

 complicated Hfe histories, often including an alternation of sexual and asexual 

 reproduction. Their hosts are animals of many types from protozoans to man. 

 As spores they commonly pass from one host to another. A spore is a young 

 individual or group of them (sporozoites), usually enclosed in a capsule, 

 capable of establishing the parasite in a new host. 



In addition to malaria in man and birds, sporozoans cause the serious dis- 

 eases of coccidiosis in fowls and rabbits, certain fevers in cattle, and the 

 pebrine disease of silkworms. 



Gregarines. The Gregarines are chiefly parasites in the body cavities of 

 invertebrates. They are common in grasshoppers, cockroaches, and in the 

 seminal vesicles of earthworms. The latter are easily examined and beautiful 

 when taken from freshly killed worms; the viscera of pickled worms are drab 

 and sterile. Pieces of earthworm vesicle can be teased out in a little water on 

 a glass slide. If they are infected, the ciliated adult parasites will swim 

 through the mealy debris and the spores containing the young parasites will 

 be scattered through it or packed in cysts. 



Coccidia. The Coccidia live in the epithelial cells of many vertebrates and 

 a few invertebrates. Their life history is complex and, in essentials, similar to 

 that of the malarial parasite. That of a coccidian (Eimeria schubergi), a 

 parasite of centipedes, is typical of others. It is swallowed with the food and 

 passes on into the intestine as a cyst (oocyst) containing several young indi- 

 viduals (sporozoites). The sporozoites enter the cells of the intestinal lining. 

 They divide repeatedly producing two kinds of individuals; asexual ones that 

 enter cells and divide asexually; and sexual ones that enter cells and enlarge 

 into egg cells (macrogametes) or enlarge and undergo multiple division into 

 sperm cells (microgametes). A micro- and a macrogamete fuse forming a 

 zygote as in ordinary sexual reproduction. The zygote surrounds itself with a 

 secretion which hardens into a shell. Within this shell or cyst (oocyst) it 

 divides several times until the cyst is packed with young parasites (sporo- 

 zoites). While this division is going on, the cyst is either in the lower intestine 

 or has been thrown outside the body. There along with millions of its kind it 

 wins or loses a chance to be ingested by another centipede. 



Hemosporidia. These sporozoans are parasites of vertebrates, blood suck- 

 ing insects and other arthropods. In the vertebrates, they inhabit the blood 

 cells and plasma. In arthropods that transmit them from one vertebrate to 

 another, they occur in the stomach and salivary glands. Those that most affect 

 human welfare are the species that cause human malaria. 



Malaria. Malaria, meaning bad air, is the name of a group of infections 

 caused by microscopic protozoan parasites (Class Sporozoa) that live mainly 



