ZOOLOGY AND BOTANY, .MICROSCOPY, ETC. 587 



It is safe to prophesy that when the known facts have been doubled 

 or trebled, the ironical statement — which now prefaces so many memoirs 

 on the Ciliates — that " the Protozoa are the simplest organisms in which 

 to study the great problems of biology " will disappear from biological 

 literature. It is vain to seek for simple mechanical factors which 

 " induce conjugation " in Ciliates. " For conjugation is the resultant 

 of many external and internal factors — environmental opportunities, 

 inherent inclinations and potencies— which are no less complex and no 

 more easily comprehensible than the factors which result in comparable 

 phenomena in man. They are really less easy to understand, because 

 we have no conception of what the ' motives ' may be which actuate a 

 brainless, non-cellular creature." 



No new light has been thrown upon the great problems of organic 

 evolution by a study of the Ciliates. Nor can we argue from them even 

 to other Protozoa. The Ciliates are so curiously organized that in many 

 ways they stand alone among organisms. What is true of Ciliates is 

 not necessarily, or even probably, true of most other organisms. 



Rat Trypanosomes in relation to the Rat-flea.* — J. D. Thomson 

 and the late E. A. Minchin investigated the rat trypanosome (Trypano- 

 soma Jewisi), and its relation to the rat-flea, Ceratophyllus fasciatus. 

 Fifty fleas were secured from rats trapped in the open, and were allowed 

 to multiply in two cages, in one of which was a " clean " and in the 

 other an infected rat to serve as food. The whole stock of fleas was 

 free from any natural infection with leptomonads or other flagellate 

 parasites, and thus a fertile source of confusion and error was avoided. 

 A full account of the structure of the flea is not given* but points of 

 interest are noted in connexion with the structure of the nervous 

 system, the reproductive system, and the heart. The histological struc- 

 ture of the stomach is described in detail. The rest of the first part 

 of the paper is devoted to summaries in tabular form of the results 

 of the different feedings, and an account of the technique employed 

 in dissection. 



The second part of the paper deals with the development of Trypa- 

 nosoma lewisi within the flea. During the whole course of development 

 it is confined to the alimentary canal proper, and is found in the 

 stomach, intestine, and rectum, never in the general cavity of the body 

 or in the salivary glands. Exceptionally, however, it may occur in the 

 rnalpighian. tubules. A stomach phase and a rectal phase can be dis- 

 tinguished. The stomach phase lasts from twenty -four hours to four or 

 even five days, and is probably always put a stop to by the second 

 feed, that is, the next after the one by which the flea was infected. This 

 phase is characterized by a peculiar mode of multiplication, the trypano- 

 somes penetrating into the epithelial cells lining the stomach, and there 

 reproducing by a process of multiple fission. The trypanosomes seen in 

 the intestine are active migratory forms on their way to the rectum, 

 where the next phase begins. Occasionally, individuals in the rectal 

 phase are found attached to the Avails of the intestine near the pyloric 

 opening, having probably migrated thither from the rectum. 



* Quart. Journ. Mior. Sci., lx. (1915) pp. 463-692 (10 pis. and 24 figs.). 



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