One of our Common Monads. 191 
Ehrenberg, about wbicb thus far but little that is satisfactory bas 
been known. 
Until recently but little bas been known of tbe real nature of 
tbe Monadina. As a natural consequence, organisms liave been 
referred to tbe different genera of this family upon very slight 
study and superficial resemblances. Urella, wbicb Ehrenberg de- 
scribes thus, “ aggregate monads, free-swimming, tailless, devoid of 
an eyespeck, and having one or (doubtfully) two flagelliform fila- 
ments,” bas of course received its share of attention, until one might 
think from tbe “ species ” and tbe figures given that tbe diagnosis 
of tbe genus bad been “anything very minute, aggregated, free- 
swimming,” — whether Infusorian or Alga making no difference. 
When, therefore, I found in a collection made at Spy Pond, near 
Cambridge (U.S.A.), on tbe 25th of November last, a large number 
of individuals of this genus (probably tbe species glauconia Ehr.), I 
made use of tbe opportunity for a careful study of it, devoting my 
spare time to it daily as long as I continued to find it in tbe water ; 
what follows is therefore tbe result of a number of observations, at 
wbicb each point has been examined and verified. 
Urella probably finds its nearest ally in Anthophysa, differing 
from that genus principally in bemg free-swimming instead of fixed 
upon a stalk. Tbe number of monads in a colony is quite variable, 
almost every number having been seen, up to forty or fifty ; in this, 
however, as in many other respects, tbe constant activity of tbe 
colonies renders it impossible to speak with absolute certainty ; even 
when cornered, so that they could no longer progress in tbe direc- 
tion in wbicb they bad 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 single monad, would be 
seen, and these were more available for purposes of study, though 
tbe larger groups were more frequent. 
From such measurements as I was able to make while they 
were in motion, I should say that tbe average length of each 
monad was about Woo th and tbe breadth Wooth of an inch ; but 
these dimensions varied a good deal with tbe size of tbe colonies, 
tbe individuals in tbe larger groups being more elongated and nar- 
rower than those in tbe smaller ones. The form may be described 
as conical, with a rounded base not at right angles with tbe axis of 
tbe cone, tbe part at tbe greatest distance from tbe apex being tbe 
one nearest tbe apex of tbe colony ; in colonies of over ten or twelve 
tbe axis of tbe cone being also bent towards tbe apex of tbe group, 
especially in those Monads near tbe base. Tbe form of tbe larger 
colonies varied from hemispherical to raspberry or even mulberry 
shape. 
I was unable to find any trace of a common investing membrane 
