Monadina.] THE INFUSORIA. 89 



300 diameters, and even this, in some cases, will be found 

 insufficient. They are besides a difficult genus to be 

 accurately determined, not only on account of their exceed- 

 ing minuteness, but because the young of animalcules of 

 other genera are so likely to be mistaken for them : for 

 instance, the young of the Bacterium, Vibrio, Uvella, 

 Polytoma, Pandorina, Gonium, &c., when they have 

 separated from their clusters, or issued from their common 

 envelopes. And this difficulty in discriminating them 

 will be more likely to happen when they are not observed, 

 whilst undergoing the process of self- division, or when 

 seen in water containing but a small number of them ; in 

 which cases, however anxious we may be to ascertain 

 their name, we must often rest contented with an approxi- 

 mation to the truth. When the water swarms with the 

 creatures, the decision will be far easier and more to be 

 relied upon, as the characters are then more easily discover- 

 able from the numerous vital relationships presented to us. 

 The observer may, however, be guided to a certain 

 extent by the following rule : Suppose that in a drop of 

 water containing species of the genus Vibrio, Bacterium, 

 Uvella, or Polytoma (easily distinguished by their clustering 

 forms), you were to observe separate Monad-like bodies ;' 

 the probability is that they would be either single forms, 

 or the young of these clustering animalcules, and if there 

 were no great difference in the size of the separate 

 individuals and those forming the clusters, this conclusion 

 would be generally correct; and this rule applies equally 

 to those green Monad-like creatures found amongst the 

 Pandorina and Gonia. The young of the Chlamidomonas 

 pulvisculus is very deceptive, and may often be mistaken 



