THE PROTOZOA 



phase of the Hfe cycle. During the aduh, attached phase of the cycle the ciHa 

 are replaced by structures called tentacles, used in feeding. Representative 

 genera are Ephelota, Podophrya, and others (Fig. 8.25). The mature animal 

 is attached to the substratum by a stalk, and its tentacles radiate from the 

 central cell body. Small organisms coming into contact with the knob-like 

 ends of the tentacles are held fast. Apparently the tentacles digest their way 

 through the surface of the captive. The fluid contents of the prey may be 

 seen later streaming down through the tentacles into the body of the suc- 

 torian, as the prey, if it is small enough to be destroyed in this manner, 

 slowly shrivels until released as a crumpled mass. Frequently a suctorian 

 attacks ciliates much larger than itself, such as Paramecuim, which is some- 

 times seen swimming with a Podophrya attached. Reproduction in suctorians 

 involves cell division of a peculiar type which is usually termed budding. 

 In this process the nuclei divide, as in Paramecium, one set of daughter nuclei 

 being pinched off with a bud of cytoplasm into a temporary cavity within the 

 distal end of the adult body. Within this cavity the bud gradually enlarges 

 and develops bands of cilia. When it is released, it swims about by means 

 of these cilia for a short time, then settles to the substrate and develops the 

 stalk and tentacles of an adult. A process of conjugation is also known for 

 the Suctoria. 



Biogenesis 



Historical. Some general problems intimately related to unicellular 

 organisms may now be examined. One such problem involves the origin of 

 living forms, or biogenesis. As a result of investigations extending over more 

 than 200 years, it was shown during the third quarter of the nineteenth 

 century that ablogenesis, or the spontaneous origin of organisms, does not 

 occur. Organisms come from pre-existing organisms by the processes of 

 reproduction. It was natural for the ancients to believe that animals such as 

 insects, which suddenly swarmed in certain places, were produced from the 

 mud of the fields under the influence of the sun's rays or arose spontaneously 

 within the decomposing carcasses in which they were found. It was even 

 supposed that mammals arose spontaneously within the female, although 

 under the influence of seminal fluid from the male. 



The higher animals were known to have parents, but the nature of the 

 continuitv between generations was not comprehended, except as the eggs of 

 birds and reptiles produced young and mammals gave birth to living oflT- 

 spring. Gradually it was recognized that smaller animals also arise from 

 eggs. The Italian naturalist, Redi, performed experiments (1688) which 

 showed how maggots originate in meat from eggs laid by flies. He placed 

 meat in jars, covering some with wire gauze and some with parchment and 

 leaving others uncovered. Flies were attracted and laid their eggs upon the 

 meat or upon the gauze. Maggots were seen to hatch from the eggs laid on 



259 



