14 PROTOZOA 



materials or stored reserves, must give rise to waste products. 

 The exchange of oxygen from without for carbonic acid formed 

 within is termed " respiration," and is distinguished from the 

 mere removal of all other waste products called " excretion." 

 In the fresh-water Amoeba both these processes can be studied. 



Respiration/ or the interchange of gases, must, of course, take 

 place all over the general surface, but in addition it is combined 

 in most fresh-water Protista with excretion in an organ termed 

 the "contractile " or " pulsatile vacuole" (Figs. 1, 4, etc.). This 

 particular vacuole is exceptional in its size and its constancy of 

 position. At intervals, more or less regular, it is seen to con- 

 tract, and to expel its contents through a pore ; at each contrac- 

 tion it completely disappears, and reforms slowly, sometimes 

 directly, sometimes by the appearance of a variable number of 

 small " formative " vacuoles that run together, or as in Ciliata, 

 by the discharge into it of so-called " feeding canals." As this 

 vacuole is filled by the water that diffuses through the substance, 

 and when distended may reach one-third the diameter of the 

 being, in the interval between two contractions an amount of 

 water must have soaked in equal to one-twenty-seventh the bulk 

 of the animal, to be excreted with whatever substances it has 

 taken up in solution, including, not only carbon dioxide, but 

 also, it has been shown, nitrogenised waste matters allied to 

 uric acid.- 



That the due interchanges may take place between the cell 

 and the surrounding medium, it is obvious that certain limits to 

 the ratio between bulk and surface must exist, which are dis- 

 turbed by growth, and which we shall study hereafter (p. 2. '5 f). 



The Protista that live in water undergo a death by " diffiu- 

 ence " or " granular disintegration " on being wounded, cruslied, 

 or sometimes after an excessive electric stimulation, or contact 

 with alkalies or with acids too weak to coagulate them. In this 

 process the protoplasm breaks up from the surface inwards into a 

 mass of granules, the majority of v/hich themselves finally dis- 

 solve. If the injury be a local rupture of the external pellicle or 



^ Energy may be derived from the mere S2)littimj up of comjilex substances 

 within the cell : when such a splitting involves the liberation of CO2 the process 

 is (mis-)called " intramolecular respiration." 



- A similar organ, but with cellular walls, is the bladder of the Rotifers and 

 certain Platyhelminthes, in connexion with their renal system (vol. ii. pp. 53, 199, 

 and e.sjiccially pp. 2l:3-.">\ 



