THE INFUSORIA 175 



(Fig. 85). Of these the euglenoids are perhaps the most common, 

 but others are frequently observed. In forms like Trachelomonas 

 it seems that the cell wall is very firm, and in others, like Peranema, 

 the cell exhibits surprising mobility. An interesting type is the 

 choanoflagellates like Monosiga, in which there is a delicate collar 

 surrounding the flagellum. Another is the marine genus Noctiluca, 

 the "nocturnal light," a very large species having one finger-like 

 flagellum, another smaller one in the region of the buccal groove, 

 and cytoplasm which is highly vacuolated. In this instance the 

 cells produ(;e brilliant luminescence throughout large areas of the 

 ocean. 



Many of the Mastigophora are parasitic. The posterior region 

 of the digestive tract in man often harbors such forms, and almost 

 any frog or tadpole will show more than one species of these para- 

 sites in its large intestine. A most interesting type is the Iry- 

 panosome (Fig. 109, p. 210), many species of which occur in the 

 blood of vertebrates and in the digestive tracts of invertebrates. 

 These are elongate cells with an undulating membrane along one 

 side, on the edge of which is attached a flagellum that becomes 

 fiee posteriorly and arises anteriorly within the cell. One famous 

 species is Trypanosoma gambiense, the parasite causing one type 

 of the sleeping sickness that is so fatal to man in equatorial Africa. 

 Among the plant-like Mastigophora there are some striking 

 examples of colonial organization which suggest a transition from 

 the unicellular to the multicellular state. Some of these are 

 considered in Chapter 8. 



The Infusoria 



Structure of Paramcecium caudatum. — The ciliated Protozoa, 

 or Infusoria (Fig. 94), are grouped into two main subdivisions: the 

 Ciliata, which possess cilia throughout the life cycle; and the 

 Sudoria, in which cilia are present for only a limited period, and 

 are later replaced by the so-called tentacles, which are used in the 

 capture of food (Fig. 84 D) . Among the CiUata, the species Para- 

 mcpcium caudatum (Fig. 95) is one of the simpler types and is a 

 common object of laboratory study. If any one kind can be 

 said to be the omnipresent protozoan of 'fresh water, it is paramce- 

 cium, since no other type occurs more commonly in cultures and 

 under a wider range of circumstances in nature. It is, so to 

 speak, adapted to " slum " conditions, and hence can be main- 



