288 THE PROTOZOA 



to be a calcium salt, which was determined as calcium orthophos- 

 phate (Ca^PO^). 1 



While there are many chances for error in Schewiakoff's work, it 

 is probable that he has come very near to the correct interpretation 

 of these bodies. Their origin, however, as well as their significance, 

 remains in doubt. Their disposal also has not been satisfactorily ac- 

 counted for. Stein reported their defecation with the undigested 

 remains through the anus, but Entz, Maupas, and Schewiakoff believe, 

 apparently on justifiable grounds, that they are dissolved and pass to 

 the outside through the contractile vacuole. 



B. RESPIRATION 



The assumption that Protozoa take in oxygen and liberate carbon 

 dioxid rests almost entirely upon indirect evidence, which, however, is 

 so strong as to leave little reason to doubt the validity of the assump- 

 tion. An infusorian, for example, moving rapidly day and night 

 during its entire life, and eating constantly throughout this period, 

 must undergo continual waste in the liberation of energy by com- 

 bustion. A constant supply of oxygen and a constant excretion of 

 the waste products of combustion appear to be equally necessary for 

 the continuance of this activity. With remarkable intuition, Spallan- 

 zani (1776) suggested the contractile vacuole as the organ of respira- 

 tion by means of which the waste matters are thrown out, and later 

 observers have offered no evidence of value to disprove the suggestion. 

 In a ciliate,. for example, the volume of a contractile vacuole at com- 

 plete diastole is about one-tenth of the volume of the animal itself, 

 and, contracting every two or three minutes, the vacuole must in half 

 an hour expel to the outside a volume of water equal to that of the 

 entire animal. The oxygen-laden water which is thus expelled must 

 have entered the body through the mouth opening or by osmosis 

 through the body walls. The important rdle which respiration plays 

 in the physiology of the Protozoa, and the agency of the contractile 

 vacuole in this process, was first clearly recognized by Schwalbe ('66) 

 and Zenker ('66), then by Wrzesniowski ('69), and later by Ross- 

 bach ('72), Biitschli ('77), Limbach ('80), Maupas ('83), and Fiszer 

 ('85). Before the period of Schwalbe and Zenker, the contractile 

 vacuole had been interpreted as a heart and the centre of a circu- 

 latory system (Corti, 1774; Gleichen, 1778; Wiegmann, '35; Siebold, 

 '48), etc., and Pouchet ('64) went so far as to describe colored blood 

 pumped by the vacuole throughout the body. This view, which now 



111 Calcium is by far the most abundant metallic element in the body. ... It is foun \ 

 in all the cells and fluids of the body, probably loosely combined with proteid." Howell, loc. 

 cit. p. 967. 



