54 Researches on the Life History of a Cercomonad : 



without a break in observation, tlirougli all their changes, and then, 

 by repeating these observations, decide on the stability or otherwise 

 of the form. For some years our attention has been individually 

 directed to this subject ; and three years since the advisability of 

 combined work commended itself to us. For work of this kind to 

 be effective, we believe there must be more than one observer, in 

 order that the observations may be unbroken as far as possible, and 

 also to secure a mutual as well as a double confirmation. 



The powers we have employed are Powell and Lealand's^V? 

 ■jV' tVj tV> h ^T^^ Eoss' -g- and \. We commenced upon a monad 

 which is at present undescribed, but which is, under some circum- 

 stances, found in enormous quantities in the fluid resulting from 

 the maceration of a cod's head. Our mode of procedure we shall 

 not now describe, as we purpose doing so at length eventually. 

 Suffice it to say that we employ an arrangement by means of which 

 a drop of the infusion under examination may be kept wet, and its 

 contained organisms preserved in life and health for an indefinite 

 length of time, when examined even with the -V. We have on 

 several occasions kept the same drop under examination, with its 

 living inmates multiplying, for from eight to fourteen days, and 

 during the whole of that time the object-glass has not been more 

 out of focus than two or three turns of the fine adjustment. 



The form on which our constant labour has been bestowed passes 

 through a remarkable series of changes, all and every one of which 

 might be taken for a distinct and independent creature, but that we 

 have tracked it through all its ti-ansitions, and seen it pass from the 

 one into the other. We find that these changes are the reverse of 

 capricious ; they are always alike. The stabihty of their recurrence 

 is as complete as that of an entomological form. But we have yet 

 one very important investigation to complete. On this we were 

 working with the highest powers, when our attention was arrested 

 by the appearance in the field of a form very similar to the one 

 with which we were so long familiar, but manifesting an entirely 

 dissimilar behaviour. The new-comer was, roughly speaking, 

 like the old form in size and shape, but instead of two flagella at 

 one end, it had a single flagellum at each end, and it was multiply- 

 ing by fission with great rapidity. Reasoning from what we knew, 

 we felt assured that this was not a capricious development of our 

 old friend whose life history we had almost compassed. But we 

 had no right to positively assume this without investigation ; and 

 the researches that ensued led to the observations now recorded. 



The field as first seen presented the appearance figured in 

 Plate XXIV., Fig. 1. Among a number of the forms on which we 

 had been working, possessing the two flagella at one end, marked a, 

 were several cercomonads with an equal flagellum at each end, 

 marked h, moving about the field with great activity ; and some of 



