118 PROTOZOOLOGY 



The Hypermastigina of termites are killed, according to Cleve- 

 land (1925), when the host animals are kept in an excess of oxygen. 

 Jahn found that Chilomonas paramedian in bacteria-free cultures in 

 heavily buffered peptone-phosphate media at pH 6.0, required for 

 rapid growth carbon dioxide which apparently brings about a favor- 

 able intracellular hydrogen-ion concentration. Respiratory metabo- 

 lism (Meldrum, 1934; Jahn, 1941). 



Excretion and secretion 



The catabolic waste material composed of water, carbon dioxide, 

 and nitrogenous compounds, all of which are soluble, pass out of the 

 body by diffusion through the surface or by means of the contractile 

 vacuole (p. 83). The protoplasm of the Protozoa is generally con- 

 sidered to possess a molecular make-up which appears to be similar 

 among those living in various habitats. In the freshwater Protozoa 

 the body of which is hypertonic to surrounding water, the water 

 diffuses through the body surface and so increases the water content 

 of the body protoplasm as to interfere with its normal function. The 

 contractile vacuole, which is invariably present in all freshwater 

 forms, is the means of getting rid of this excess water from the body. 

 On the other hand, marine or parasitic Protozoa live in nearly iso- 

 tonic media and there is no excess of water entering the body, hence 

 the contractile vacuoles are not found in them. Just exactly why 

 nearly all euciliates and suctorians possess the contractile vacuole 

 regardless of habitat, has not fully been explained. It is assumed that 

 the pellicle of the ciliate is impermeable to salts and slowly permeable 

 to water (Kitching, 1936) or impermeable to water, salts and prob- 

 ably gases (Frisch, 1937). If this is the case with all ciliates, it is not 

 difficult to understand the universal occurrence of the contractile 

 vacuole in the ciliates and suctorians. 



That the elimination of excess amount of water from the body 

 is one of the functions of the contractile vacuole appears to be be- 

 yond doubt judging from the observations of Zuelzer (1907), Finley 

 (1930) and others, on Amoeba verrucosa which lost gradually its con- 

 tractile vacuole as sodium chloride was added to the water, losing 

 the organella completely in the seawater concentration and of Yo- 

 com (1934) on Paramecium caudatum and Euplotes patella, the con- 

 tractile vacuoles of which nearly ceased functioning when the ani- 

 mals were placed in 10 per cent sea water. Furthermore, marine 

 amoebae develop contractile vacuoles de novo when they are trans- 

 planted to fresh water as in the case of Vahlkampfia calkinsi (Hogue, 

 1923) and Amoeba biddulphiae (Zuelzer, 1927). Herfs (1922) studied 



