The Protozoa 51 



forward in a thin' layer coincides with the progress of the animal, 

 which continually rolls over and over as it were. 



In Mastigophora the movement of the more rigid bodies is effected 

 through the presence of longer or shorter, flexile or rigid, coarse 

 threads or "whips." These usually project anteriorly — Trypano- 

 soma — and by means of a spiral movement draw the cell along with 

 a propeller-like action; symmetrically arranged flagella may operate 

 more like oars. 



The Sporozoa usually manifest very little movement, yet their 

 sporozites are motile, and the spermatozo'ites are also motile and 

 commonly flagellated. 



The infusoria are actively motile through abundant fine hair-hke 

 formations known as cilia. These, multitudinous as they are, 

 vibrate synchronously with an oar-like movement, propelling the 

 organisms forward or backward or making them revolve with great 

 rapidity. Independent cilia not infrequently encircle the oral 

 aperture, causing a vortex, in which the minute structures upon 

 which the creatures feed are caught and carried into the body. 



Size. — The protozoa show very great variation in size. Some of 

 the Sporozoa form minute parasites of the red blood-corpuscles or 

 other cells of the vertebrates. The Treponema is so small that it 

 can slowly find its way through the pores of a Berkefeld filter. 



On the other hand, the Sarcosporidium is so large that one of its 

 cysts, composed of a single organism, can be seen with the naked 

 eye. Certain protozoa that play no part in morbid processes — 

 Myxosporidia — and so do not come within the scope of this work, 

 may be several centimeters in diameter. 



Reproduction. — The reproduction of the protozoa takes place both 

 asexually and sexually. It may be that there are no strictly asexual 

 protozoa, nearly all forms having been shown upon intimate ac- 

 quaintance to be subject to occasional conjugation. Conjugation 

 may result in the loss of individual identity or the conjugated 

 individuals may again separate. 



Whether the reproduction takes place asexually without con- 

 jugation or sexually after conjugation, it always occurs by division, 

 which may be simple and binary or complex and multiple. 



Wherever a distinct nucleus can be found, the multiplication of the 

 protozoa is preceded by some kind of mitotic change. The more 

 complex the structure of the nucleus, the more complicated and 

 perfect the mitosis. 



The elongate protozoa divide lengthwise, which is sometimes 

 contrary to expectation, as in the cases of Treponema and 

 Spirochaeta. 



The multitudinous sporozoites into which the zygotes of the 

 sporozoa divide are commonly the result of anterior division into 

 intermediate bodies know as oocysts, ookinetes, sporocysts, etc. 

 The nuclear substance is first divided so as to be 'uniformly dis- 



