Natural History of the Infusoria. 329 



Oxytricha caudata, Ehrbg. (tab. xl. fig. xi.), although I will not 

 say that they are perfectly identical; the posterior extremity in 

 particular is always bent round in the manner represented by 

 Ehrenberg at No. 3 of the figure just cited. 



Euplotes Charon, Ehrbg._, contracts itself within its shield- 

 shaped carapace into a ball, which then invests itself with a new 

 membrane. As long as the carapace, which is distinguished by 

 its striae, is still retained, there can be no doubt as to the animal 

 enclosed in the cyst. The cysts of both these Infusoria are, as 

 might be expected, very common in infusions, and are probably 

 often confounded with the cysts of Vorticella. 



Pontotrichum lagenella forms a cyst like that of Trachelius 

 described by Cohn [op. cit. supra, p. 267), which completely re- 

 tains the flask-like form of the body. Within this the animal 

 contracts itself into a ball and invests itself with a new mem- 

 brane. Amoeba also actually has a state of rest. I observed it 

 become round on one side, on which a firm membrane was then 

 formed, whilst the other portion continued its peculiar move- 

 ments. By degrees the membrane extends itself over the whole 

 body, the moveable portion constantly becoming smaller, until 

 at last a completely closed cyst is produced, in the clear interior 

 of which a round nucleus with a reddish halo, exactly like that 

 of Polytoma and other Monadina, may be distinctly observed*. 



During our investigation of Polytoma, we have always tacitly 

 regarded it as of animal nature. But if we consider how very 

 difficult it is, in the present state of our knowledge, to draw the 

 boundary-line between the animal and vegetable kingdoms, it 

 becomes necessary to inquire with what right we have done so. 

 If the cycle of development of Polytoma be completed by the 

 forms now known, it is clear, in the first place, that Polytoma 

 behaves very like a simple cell. A structureless membrane in- 

 vests a soft, membraneless substance, which is continued exter- 

 nally in the form of a pair of filaments. The nucleus behaves 

 like a cell-nucleus. It is true that if it be necessary that the 

 nucleus of an animal cell should be a vesicle, the nucleus of 

 Polytoma does not fulfil this condition. But is the proper mem- 

 brane a necessary element of the animal cell-nucleus ? Is it 

 not possible that this may be formed only under certain circum- 

 stances ? In the nucleus of Amoeba, I have often observed, on 

 the outer surface of the reddish halo, granulations which united 

 to form a closed membrane ; whilst at other times the nucleus 

 exactly resembled that of Polytoma. 



* I take this opportunity of caUing attention to this nucleus, which, 

 as far as I am aware, has not yet been noticed. By the comparison of 

 numerous specimens, its constant appearance will distinguish it from enclosed 

 particles of food. It occurs in A. diffiuens and radiosa. 



