340 



HISTOLOGY 



manner is drained from the cell. Griffiths, in 1889, showed that this 

 fluid is charged with uric acid. This type of excretory organ is common 

 to all unicellular organisms in which there is a comparatively great 

 activity. In other words, where much energy is displayed, many waste 

 products are formed, and an excretory cell structure has arisen to handle 

 them. Where the activity carried on by the unicellular form is low there 

 seems to be little or no necessity for such structure, the general surface 

 of the cell serving as a medium for the discharge of the waste products. 

 Such a contrast is seen in the gametes of certain Algae. The active 

 male gamete has one or two contractile vacuoles and the passive female 

 gamete has none. 



An example of contractile vacuoles. The contractile vacuoles of 

 Paramcecium aurelia or Paramcecium caudatum are always two in num- 

 ber. Unlike food vacuoles, their number is constant, and they are sta- 

 tionary; also, they are permanent features of the cell. Their inner 



FIG. 302. Individual of Paramoecium caudatum. Arrows show course of food vacuoles (f.v.*). 

 nu., nuclei; con.v., contracting vacuoles, one empty and one full; f.m., fecal matter; tr., 

 discharged trichocyst. X 375. 



surface dips into the endoplasm and their outer surface opens through 

 the ectoplasm to the exterior. Into each contractile vacuole a radiating 

 series of drainage channels lead (Fig. 302). These channels empty 

 their contents into the contractile vacuole. The channels are filled as 

 the contractile vacuole discharges its contents. These vessels are best 

 seen when filled. So the channels become conspicuous as the vacuole 

 becomes indistinct. 



In the ccelenterates there are two factors which in part account for 

 the absence of specialized excretory structures. First,' the activity of 

 these forms is low, resulting in the formation of a relatively small amount 

 of waste products. And secondly, all of these cells except the very 

 passive mesodermal cells have a surface exposure. It is quite probable, 

 therefore, that the surface exposure in the ccelenterate body is sufficient 

 to remove the relatively small amount of urine evolved. 



Waste products of the internal cells are thrown off by these cells 

 into the intercellular mesenchymal spaces to be passed to the epithelia 



