I I 6 PROTOZOA 



cells have come to rest, but in most cases it occurs between 

 active cells. We find every transition between equal unions and 

 differentiated sexual unions, as we shall see in discussing the 

 Volvocaceae.^ The " coupled-cell " differs in behaviour in the 

 different groups, but almost always goes to rest and encysts at 

 once, whatever it may do afterwards. 



The life-history of many Flagellates has been successfully 

 studied by various observers, and has shed a flood of light on 

 many of the processes of living beings that were hitherto 

 obscure. The first studies were carried through by the patient 

 labours of Drysdale and Dallinger. A delicate mechanical stage 

 enabled the observer to keep in the field of view a single 

 Flagellate, and, when it divided into two, to follow up one of the 

 products. A binocular eye-piece saved much fatigue, and 

 enabled the observers to exchange places without losing sight of 

 the special Flagellate under observation ; for the one who came 

 to relieve would put one eye to the instrument and recognise the 

 individual Flagellate under view as he passed his hand round to 

 the mechanism of the stage before the first watcher finally 

 relinquished his place at the end of the spell of work. Spoon- 

 feeding by Mrs. Dallinger enabled such shifts to be prolonged, 

 the longest being one of nine hours by Dr. Dallinger. The life- 

 cycles varied considerably in length. It was in every case found 

 that after a series of fissions the species ultimately underwent 

 conjugation (more or less unequal or bisexual in character) ; ^ 



^ And also in the "Monads," described by Dallinger and Drysdale, see above. 



^ In Cercomonas dujardinii, Polytoma uvella, and Tetramitus rostratus tlie 

 gametes resemble the ordinary forms and are isogamous. In Monas dallingeri and 

 Hodo caudalus conjugation takes place between one of the ordinary form and size 

 and another similar but smaller. In Dallingeria drijsdali the one has the ordinary 

 size and form, the other is equal in size, but has only one flagellum, not three ; in 

 Dodo saltans they are unequal, the larger gamete arising in the ordinary way by 

 longitudinal fission, the smaller by transverse division. Doubt has been thrown 

 on the validity of our authors' results by subsequent observers abroad ; but I can 

 find no evidence that these have even attempted to repeat the English observations 

 under the same severely critical conditions, and therefore consider the attacks so 

 far unjustified. Schaudinn has observed conjugation between Trichomonas indi- 

 viduals which have lost their flagella and become amoeboid ; also in Lamblia intes- 

 tinalisB.nd in Trij]}anosoma (Halteridium ?) noctuae {Vig. 39) "Reduction-divisions"' 

 (see p. 75, note 1) of the nuclei take place before fusion, and the nuclear pheno- 

 mena are described as "complicated " (Arh. ICais. Gesundheitsamte, xx. 1904, p. 387). 

 Paramneba eilhardii in its adult state is colourless, amoeboid, multiciliate. It forms 

 a brood cyst, from which are liberated flagellate zoospores, with a chromatophore, 

 which reproduce by longitudinal fission in this state. They may also conjugate. 



