ONE OF OUR COMMON MONADS. 287 
`- Urella probably finds its nearest ally in Anthophysa, differing 
from that genus principally in being free-swimming instead of 
fixed upon a stalk. The number of monads in acolony is quite 
variable, almost every number having been seen, up to forty or 
fifty; in this, however, as in many other respects, the constant 
activity of the colonies renders it impossible to speak with abso- 
lute certainty ; even when cornered so that they could no longer 
progress in the direction in which they had been moving, they 
continued to revolve upon their axes with considerable rapidity, 
making it impossible to count them with accuracy. Occasionally, 
a group of five or six or even two or three and not unfrequently a 
Fig. 89. 
An ideal section through a colony of Monads. 
single monad would be seen, and these were more available for 
purposes of study, though the larger groups were more frequent. 
From such measurements as I was able to make while they were 
in motion, I should say that the average length of each monad 
was about one two-thousandth and the breadth one five-thousandth 
of an inch; but these dimensions varied a good deal with the size 
of the colonies, the individuals in the larger groups being more 
elongated and narrower than those in the smaller ones. The form 
may be described as conical with a rounded base not at right an- 
gles with the axis of the cone, the past at the greatest distance 
from the apex being the one nearest the apex of the colony ; in 
Colonies of over ten or twelve the axis of the cone being also bent 
towards the apex of the group, especially in those monads near 
the base. ‘The form of the larger colonies varied from hemispher- 
