The life -history of Sarcocystis tenella, a Cnidosporidian, will be considered. It is a parasite of vertebrates, sometimes occurring as a cystic white body, known as "Miescher's tubes," visible to the naked eye in the striped muscles of man. As a rule it produces no symptoms. The cyst contains a number of crescentic bodies (spores); in some cases these spores are motile; each spore is provided with a nucleus situated at the blunt posterior end and a striated polar capsule at the anterior end. In this species the mode of propagation from one host to another is unknown, but mice can be infected by feeding on the bodies of others harbouring S. muris; apparently in this case the spore germinates in the intestine of its host and sets free an amœbula which eventually reaches the striped muscle. In this situation the nuclei break up to form a plasmodium, and become the central points of many separate cells, called pansporoblasts or sporonts. These sporonts multiply actively and occupy the peripheral zone of the parasite, whereas the central portion is packed with spores in the process of differentiation.
Of the Haptosporidia, one, Rhinosporidium kinealyi, is found in man in India. It gives rise to polypoid outgrowths from the septum nasi and from the external auditory meatus. The youngest parasites appear to be round cells provided with a cuticle and a single nucleus. By repeated division of this nucleus the parasite becomes a multinucleate plasmodium. The cyst contents become divided into pansporoblasts in the centre, while the periphery contains the granular nuclei. The sporonts grow in size, and by repeated division form clusters of spores, a kind of a morula enclosed in a membrane. When ripe the cyst bursts and scatters its contents into the surrounding tissues.
Nothing is known of the transmission of this parasite or the manner in which infection is acquired.
Class IV.—INFUSORIA
The Infusoria are represented by Balantidium coli, an oval-shaped parasite belonging to the subclass Ciliata. The parasite measures about 60-100 μ in length by 50 μ in breadth. The body is clothed with a thick covering of cilia. There is a lobulated nucleus with a distinct micronucleus lying close to it. The protoplasm contains a variable number of contractile vacuoles; there is a definite anteriorly-situated cystosome or mouth cavity. Nutrition is effected by ingestion of solid particles and by osmosis. The parasite reproduces itself asexually by transverse fission. Conjugation takes place by approximation of two individuals and by the exchange of certain nuclear elements; after this has been effected the conjugants separate. Encystment may take place.
Balantidium coli burrows into the submucosa and causes dysenteric symptoms known as balantidial dysentery (see p. 523).
