THE INFUSORIANS. 31 



of Protoplasta. Other Gregarinae are very minute, and are 

 parasitic in insects (Fig. 19), etc., and vary greatly in form, 

 some being apparently segmented, while in a few forms the 

 body ends anteriorly in a sort of beak armed with recurved 

 horny spines. We are now prepared to adopt the following 

 definition of the class : 



CLASS III. GREGARINIDA. 



Amceba-like Protozoa, more or less elongated, with a determinate cell- 

 wall, with a subcuticular system of muscular fibrilla, with a nucleus, but no 

 contractile vacuole ; reproducing by encysting and subdivision of the cen- 

 tral mass of the body, producing shelly psorosperms, from which escape the 

 moner-like young, which undergo a metamorphosis into the usually worm- 

 shaped, parasitic adult (Gregarina). 



CLASS IV. INFUSORIA. 



These organisms can best be understood by studying rep- 

 resentatives of the three orders forming the class.* 



Order 1. Flngellata (Monads). A familiar example of 

 monads, Oikomonas termo Clark, has been studied by H. 

 J. Clark. His description will suit our purpose of indi- 

 cating the form and habits of a typical flagellate animalcule. 

 It somewhat resembles our figure of Uvella in its general 

 shape, being pear-shaped, faint olive in color, and provided 

 with a vibratile locomotive lash or flagellum. In swimming, 

 the monad stretches out the flagellum, which vibrates with 

 an undulating, whirling motion, and produces a peculiar 

 graceful rolling motion. When the monad is fixed the fla- 

 gellum is used to convey food to the mouth, which lies be- 

 tween the base of the flagellum and beak, or " lip." The 

 food is thrown by a sudden jerk, and with precision, directly 

 against the mouth. " If acceptable for food, the flagellum 

 presses its base down upon the morsel, and at the same time 

 the lip is thrown back so as to disclose the mouth, and then 

 bent over the particle as it sinks into the latter. When the 

 lip has obtained a fair hold upon the food, the flagellum 

 withdraws from its incumbent position and returns to its 

 former rigid, watchful condition. The process of degluti- 



* Kent's Manual of the Infusoria, London, 1880 ; Stokes' Microscopy 

 for Beginners, 1887. 



