176 UNICELLULAR ANIMALS. 



macrogamete. Witliin the body of the latter, after complicated 

 changes, the nuclei fuse together, and this is followed by fission. 

 The analogy of conjugation to the fertilization of the Qgg is here 

 complete. The conjugating cells show a sexual differentiation, 

 one being like the ovum, large and fixed, the other like the 

 spermatozoon, small and motile. 



As in Paramoscium the macronuclei entirely disappear, fusion takes 

 place between derivatives of the micronuclei, and from the resulting body 

 both macronuclei and micronuclei are derived. 



Euglena and Other Simpler Infusoria. Besides forms like 

 Parammciiim and Yorticella which bear numerous cilia, there 

 are many Infusoria which possess only one large lash orflagellum. 

 Of these Euglena^ which is sometimes found in stagnant water, 

 sewage-polluted pools, etc., is one of the most interesting, inas- 

 much as it contains chlorophyll, possesses an " eye-spot " of red 

 pigment, and under certain conditions exliibits amoebiform 

 movements. 



Compound or " Colonial '' Forms. In a number of forms, 

 closely related to Yorticella, the individuals (" zooids ") formed 

 by fission do not immediately separate, but remain for a time 

 united to form a ' ' colony ' ' which may contain hundreds of 

 zooids. Zoothammo7i, a common species, thus forms a beautiful 

 tree-like organism, consisting of a single central stalk with nu- 

 merous branching offshoots from its summit, each twig terminat- 

 ing in a zooid. The entire system of branches is traversed by a 

 continuous contractile axis. Carchesiurrh is similar, but the axis 

 is interrupted at the beginning of each branch. In Epistylis 

 the entire axis is non-contractile. 



Such colonial forms are of high interest as indicating the 

 manner in which true multicellular forms may have arisen. 

 From the latter, however, they differ not only in the fact that 

 the association of the cells is not permanent, but in the absence 

 of any division of labor among the units. 



Physiology. Most Infusoria are true animals, agreeing with 

 Ainoeba in the essential features of their nutrition, and having 

 the power to digest not only proteids, but also carbohydrates and 

 fats. ParmnoeciuTn and Yorticella are herbivorous forms, 

 feeding upon minute plants, and especially upon the bacteria- 



