THE GENERAL PHYSIOLOGY OF THE PROTOZOA 197 



products of metabolism, a primary necessity of anaerobic life. The question 

 of size is also a factor, since deleterious substances may diffuse from the 

 surface of the body, and in a small body the surface is greater in proportion 

 than in a larger one. Consequently the conditions are more favourable for 

 a smaller species, such as Paramecium, than for a large form, such as 

 Spirostomum. 



Excess of oxygen was found by Putter (198) to have an injurious effect 

 on Spirostomum, affecting, however, only the cytoplasm, and not the nucleus, 

 in the first instance. 



On the current view that the symbiotic vegetable organisms present in 

 many Protozoa aid in the respiratory processes by absorbing the carbon 

 dioxide, breaking it up, and setting free the oxygen, the experiments of 

 Lipska (173) on a culture of Paramecia which contained green algae (Proto- 

 coccaceae) in their endoplasm are of considerable interest. In two glass 

 vessels of equal size there were placed, in the one Paramecia with, in the other 

 without, the algae in their body. Hydrogen was circulated through the vessels 

 to drive out the air, after which they were hermetically sealed and exposed 

 to the same conditions of light and temperature. After fifty hours the 

 vessels were opened. The Paramecia without algse were dead, but those 

 containing algse were still alive, though feeble in their movements, and they 

 revived completely in about twenty-four hours after air had access to them. 

 In another experiment two batches of Paramecia were kept in the dark ; 

 after eight days those without algae were dead, while those containing algse 

 were perfectly normal. Old cultures of Paramecia containing algae showed 

 no conjugation ; Lipska explains this as due to the influence of the algae, 

 since, by setting free oxygen, they prevent the development of anaerobic 

 bacteria which produce substances toxic to the Infusoria. 



According to Popoff (185), the depression- periods of Protozoa (p. 208) are 

 partly due to derangements of the respiratory processes and to accumulation 

 of products of metabolism in the cell. 



3. Excretion and Secretion. The waste substances excreted from 

 the protoplasm may be either soluble or insoluble in nature. If 

 soluble, they may either pass out of the protoplasmic body by 

 diffusion from the surface, or may be removed by the agency of the 

 contractile vacuoles. 



Contractile vacuoles are of common occurrence in free-living fresh-water 

 Protozoa, but are usually wanting in marine forms, or, if they occur in them, 

 they pulsate very slowly. They are generally absent also in entozoic and 

 parasitic Protozoa, but are found, however, in some internal parasites for 

 example, in all Anoplophryince (p. 452 ; Cepede, 831). 



Some authors (e.g., Degen, 154) have described an investing membrane 

 to the contractile vacuole, but it is practically certain that no such membrane 

 exists, and that the vacuole is simply a drop of watery fluid lodged in, and 

 bounded by, the more viscid protoplasm, without any special structural 

 differentiation (compare Khainsky, 170'5). The contractile vacuoles were 

 believed at one time to empty themselves internally, and to function simply 

 as circulatory organs ; but in all cases in which they have been studied care- 

 fully, it has been proved that they empty themselves to the exterior (compare 

 Jennings, 167, Khainsky, 170'5). 



The effect of changes of temperature is noted below (p. 206). Increased 

 pressure makes the pulse slower (Khainsky, 170'5). Degen (154), experi- 

 menting with Glaucoma colpidium, found that oxygen produced at first an 

 increase in the frequency of the pulse, which soon became normal again. 

 Hydrogen and carbon dioxide diminished the frequency and caused a dilata- 

 tion of the vacuole ; both these gases were lethal in their effect, especially 

 carbon dioxide. Isotonic solutions of neutral salts had a retarding effect. 



