STRUCTURAL DIFFERENTIATIONS 161 



in Pycnothrix monociistoides and Diplodinium ecavdafinii, 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 diff'erentiation. 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. 108) 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; two as a rule in the 

 phytoflagellates, one in Hypotrichida and Peritrichida, and variable 

 mnnbers in the Holotrichida 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 or ten radiating canals form a characteristic rosette about 

 each of the two contractile vacuoles (Fig. 85). In the Hypotrichida 

 there are usually two such canals leading to the dorsally placed 

 vacuole, and two in Sfrnfor, one foHowing the margin of the body 

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

 circular course around the body (Fig. 74). In Ojjhri/oglrna flam 

 there may be as many as thirty fine feeding canals leading from all 

 parts of the body to the centrally-placed vacuole and in Frontonia 

 leucas eight to twelve such canals follows a tortuous course through- 

 out 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 and Dino- 

 flagellates. 



In corticate Protozoa the contractile vacuole usually opens to 



the outside in the ^'icinity 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 \Tstibule of forms like Vorticella. 



11 



