170 BIOLOGY OF THE PROTOZOA 



terior region of.the body, which is closed and invisible except during 

 the process of defecation (Fig. 44, C,p. 86). In some forms, notably 

 in Pycnothrix monocystoides and Diplodinium ecaudatum, a definite 

 anal apparatus is developed. In the latter case Sharp describes a 

 " rectum" with distinct walls opening to the outside by a permanent 

 cytopyge, while at the inner end there is a "cecum" which acts as 

 a collecting vacuole for the fecal matter (Fig. 2, p. 20). 



(e) Contractile Vacuoles.— In the rhizopods and most of the soft- 

 bodied flagellates the contractile vacuole can scarcely be called a 

 cortical differentiation. In these cases they are more or less casual 

 organoids, moving freely with the endoplasmic granules. In the 

 corticate flagellates and ciliates, however, there is a permanent 

 spot in the cortex through which the contents of contractile vacuoles, 

 fixed in position, are emptied to the outside. As a rule, the salt water 

 forms of Protozoa do not have contractile vacuoles (see p. 176) and 

 the number in fresh water forms is variable, sometimes in the same 

 organism (testate rhizopods and Heliozoa). In many types, how- 

 ever, the number as well as the position is fixed; one, as a rule, in 

 Hypotrichida and Peritrichida, and variable numbers in the Holo- 

 trichida and Heterotrichida. 



In rhizopods the roving vacuole adds to its volume by picking up 

 fluid substances from all parts of the endoplasm until it becomes too 

 heavy to be easily moved with the flowing endoplasm. The vacuole 

 is thus gradually left behind, so to speak, until it finally breaks 

 through the thinning wall of protoplasm and empties its contents 

 to the outside, usually at that part of the body which for the time 

 being is posterior. In the fixed forms of vacuoles the fluids to be 

 excreted are brought to the excretory organoid by more or less 

 definite routes or canals, through the endoplasm. Such canals are 

 highly characteristic of many types of ciliates. A familiar example 

 is afforded by the different species of Paramecium where the five 

 to ten radiating canals form a characteristic rosette about each of 

 the two contractile vacuoles (Fig. 95). In the Hypotrichida there 

 are usually two such canals leading to the dorsally placed vacuole, 

 and two in Stentor, one following the margin of the body to the 

 "foot," the other following the rim of the peristome in a circular 

 course around the body. In Ophryoglena flava there may be as many 

 as thirty fine feeding canals leading from all parts of the body to 

 the centrally placed vacuole, and in Fronton in leucas eight to twelve 

 such canals follow a tortuous course throughout the body substance. 

 In Pycnothrix the canals form a branching network through the 

 endoplasm. Such canals are replaced by a ring of feeding vacuoles 

 in many of the corticate flagellates. 



In corticate Protozoa the contractile vacuole usually opens to 

 the outside in the vicinity of the anus when such a structure is 

 present. In many cases it opens into the cytopharynx as in the 

 majority of flagellates or in the vestibule of forms like Vorticella. 



