6 y THE PROTOZOA 



At this period, although the term cell had already been used by 

 Robert Hooke (1665), the idea of simplicity of organization, apart 

 from minuteness of the organs, was unknown, and until the cell- 

 theory was established in 1838, the Protozoa were regarded as com- 

 plex animals having all of the parts and organs, although of micro- 

 scopic size, found in Metazoa. Leeuwenhoek allowed his imagination 

 to see what his eyes could not. " When we see," said he, " the sper- 

 matic animalcula [spermatozoa] moving by vibrations of their tails, we 

 naturally conclude that these tails are provided with tendons, muscles, 

 and articulations, no less than the tails of a dormouse or rat, and no 

 one will doubt that these other animalcula which swim in stagnant 

 waters [Protozoa], and which are no longer than the tails of the sper- 

 matic animalcula, are provided with organs similar to those of the 

 highest animals. How marvellous must be the visceral apparatus 

 shut up in such animalcula ! " * 



The minute size of the Protozoa made it impossible for the early 

 investigators with their crude instruments, to follow out any life-cycle, 

 and the prodigious numbers and the sudden appearance of certain 

 forms in stagnating waters led to the belief already current in respect 

 to other forms, that they arose de novo. Two misconceptions thus 

 sprang up almost at the beginning of our knowledge of the Protozoa : 

 one, that Protozoa are provided with organs like higher animals ; and, 

 two, that they arise by spontaneous generation ; and one of the main 

 tasks of research on the Protozoa down to our own times has been 

 the correction of these early errors. 



It is not strange that Leeuwenhoek and his immediate followers 

 considered Protozoa as complicated organisms. Organisms without 

 organs were as novel to them as animals without cells would be to 

 us, and they described only what experience had taught them to 

 expect. With increasing knowledge of many forms and with con- 

 stantly improving microscopes, the conception of simplicity of organ- 

 ization gradually gained ground until Dujardin, about 1840, denned 

 the Protozoa as simple, slightly differentiated structures composed of 

 a fundamental living substance to which he gave the name of sarcode. 



Despite the crudity of their instruments, the early microscopists 

 obtained wonderful results. Leeuwenhoek himself, although study- 

 ing these low forms only incidentally, gave recognizable descriptions 

 of twenty-eight species, and in addition, noted the rapid increase of 

 some of the larger forms, saw conjugation or the temporary union of 

 two individuals, and discovered so-called embryos. The early litera- 

 ture soon became crowded with notices of new and interesting forms, 

 found in all sorts of hitherto unthought-of localities. Forms with 



1 Quoted from Dujardin ('41), pp. 21, 22. 



