54 



THE PROTOZOA 



they are in open connection. The canal system, which some observers 

 (e.g. Fabre Dumergue, '90) consider widely spread throughout the 

 Ciliata, is often strikingly developed, as in Frontonia, where there is a 

 complicated network traversing the entire cell (Fig. 21). 



While the excretory function of the contractile vacuole is generally 

 accepted, there have been only a few satisfactory experiments to 

 demonstrate it, and the possibility of other functions is not excluded. 

 At the present time the balance of evidence is in favor of the view 

 that the contractile vacuole has both excretory and respiratory func- 

 tions, inasmuch as it regularly empties a fluid to the outside, which 

 carries with it the products of destructive metabolism in the form of 

 urea, and probably carbon dioxid, although the respiratory function 

 has never been demonstrated. 1 



Whatever may be the function of the contractile vacuole, it does 

 not appear to be universally necessary for the life of the organism, 

 for it is lacking in the Sporozoa and the majority of the Sarcodina 

 (Reticulariida and Radiolaria). Furthermore, whatever the use of 



the vacuole, it is independent of the 

 nucleus, non-nucleated fragments form- 

 ing new vacuoles which pulsate rhyth- 

 mically for some time. Hofer ('89) 

 found that vacuoles in non-nucleated bits 

 of Amceba proteus would contract for 

 fourteen days. He also noted that 

 whereas the regular period of pulsation 



c '^K&^'&vtt^&ssg*^ was seven mmu tes, the periods became 



A longer and longer, until at the end of the 



fourth day there was but one pulsation 

 every two hours, and even then the con- 

 tents were not completely expelled, a 

 reaction which Penard ('90) formulated 

 later by the statement that the activity 

 of the contractile vacuole is directly 

 proportional to that of the entire indi- 

 vidual. 



Fig. 22. Division of Euplotes. [FROM 



A PREPARATION.] 

 The daughter-cells are almost ready 

 to separate ; the daughter-micronuclei 

 (n) are re-formed ; the macronucleus 

 (/) is not quite divided; the gastric 

 vacuoles (/) are equally distributed in 

 the two daughter-cells, one of which has 

 generated the adoral zone (az). 



3. Reproduction. 



With the exception of the Sporozoa, 

 simple division, or splitting into two 

 parts, is the characteristic mode of re- 

 production in all Protozoa. In the 

 Sporozoa, and at times in most of the 

 other Protozoa, division is replaced by 



Chapter IX, p. 283. 



