126 PHYSIOLOGY OF PROTOZOA 



They may become crystalline excretory crystals, or remain as amorphous 

 masses. Amongst the Sporozoa, when reproduction by schizogony takes 

 place, a certain amount of cytoplasm is usually left over as a residual body, 

 which takes no part in the formation of merozoites. In it is got rid of 

 a certain amount of excretory substance. Malarial parasites thus dis- 

 charge the pigment granules which accumulate as a result of digestion 

 of haemoglobin. 



In addition to the substances which have been referred to, and which 

 may be regarded as steps in the formation of protoplasm or the waste 

 products from the food, there occur other substances which are elaborated 

 to fulfil some special function. The conspicuous so-called chromidial 

 body of shelled amoebae may have to do with the formation of the shell. 

 The various skeletal structures which occur in the cytoplasm of Radiolaria, 

 the supporting rods which form the axes of the pseudopodia of many 

 Heliozoa, and, indeed, the external coverings like the shells of Foraminifera 

 and the cyst walls themselves, are to be regarded as products of metabolism. 

 It is evident that the Protozoa which produce such structures must absorb 

 special substances for the purpose. 



Quite apart from the excretion of substances no longer required by the 

 organism by the rupture of vacuoles containing them at the surface of 

 the body, there is another method of excretion, which is carried out by a 

 rhythmically contracting vacuole which is situated near the surface of the 

 body. Such a contractile vacuole, when fully formed, suddenly contracts, 

 so that the clear liquid contents are discharged through the surface of the 

 body. In a short time the vacuole re-forms, and, gradually increasing in 

 size, reaches its maximum, when it again contracts. In some cases definite 

 channels in the cytoplasm conduct fluid to the vacuole. The rate of 

 pulsation varies with temperature and the presence of substances which 

 affect the density of the medium. It is supposed that the vacuole is a 

 means of discharging carbon dioxide and other soluble excretory substances, 

 but the fact that contractile vacuoles are absent in marine Protozoa 

 and. many parasitic forms, and that fresh- water forms lose the contractile 

 vacuole when made to live in salt water, suggests that such a vacuole may 

 be a means of accommodating the organism to the medium in which it lives, 

 rather than an organ primarily excretory in function. It can hardly be 

 supposed that marine or parasitic forms are less dependent on excretion 

 for their existence than those which live in fresh water. It has been con- 

 jectured that the contractile vacuole may counteract the tendency of the 

 cytoplasm to become overcharged with water due to the greater absorption 

 in fresh than in saline water. 



On the method of nutrition of any particular organism depends the 

 character of the medium in which it can be cultivated. Forms like 



