98 
On Euchlanis triquetra and E. dilatata. 
of the Euchlanidota would disappear ; for as all its species consist 
of free-swimming loricated rotifers, it would rank with the 
Brachionsea. 
But the reasons that induced Ehrenberg to make a separate 
family of the Euchlanidota are worth the considering, as they will 
naturally lead me to speak of some of the curious peculiarities of 
the family as a whole, and especially of the two species of the genus 
Euchlanis, which more properly form the subject of this paper. 
Ehrenberg’ s imperfect instrument led him to believe that he 
could divide the trochal disks of the rotifers into four well-marked 
groups, viz. 1st, those with a single ciliated edge ; 2nd, those 
with many ciliated protruberances ; 3rd, those with two equal 
ciliated circlets ; and the first was again subdivided into single 
ciliary edges with some of the cilia wanting, and perfect single 
ciliary edges. 
Now it may be said briefly that the whole of this scheme is 
worthless. The modern instruments have shown not only that the 
trochal disks are often of a highly-complicated structure, and that 
they possess subsidiary lines of minute cilia, of the existence of 
which Ehrenberg was quite unaware ; hut also that the forms 
of the disks pass so gradually from the one into the other that it is 
hopeless to attempt to ground any system of classification upon 
them. I may add to this also that it is the most difficult thing in 
the world to determine in almost any rotifer (except the Tube- 
formers) what is the real form of the trochal disk. In confirmation 
of this statement I have only to call attention to the drawings of 
even such excellent observers as Leydig, Cohn, and Crosse, in which 
very frequently a row of lines drawn at random follows the outline 
of the head of the rotifer without the least attempt having been 
made to depict the real arrangement of the cilia. 
Now, according to Ehrenberg’s notions, the Euchlanidota had 
the trochal disk of the Hydatinea, while themselves encased in 
loricae like the Brachionea : so he formed them into an inde- 
pendent family, and at the same time pointed out that the great 
majority of the typical genus Euchlanis had a lorica slit longi- 
tudinally down the ventral surface. 
As to the trochal disk of Euchlanis , I have spent no little time 
and patience in endeavouring to understand it, and I think tbat the 
disk of Enxchlanis triquetra is correctly given at (a), Fig. 1, 
Plate XXVIII. If so, it does not much resemble that of 
Hydatina, in which there are, broadly speaking, two horse-shoe 
shaped rows of cilia one within the other. 
If anyone would like to test at once the quality of his temper 
and his microscope, I strongly recommend him to study the trochal 
disk of one of these creatures from a living specimen, and with the 
full determination of thoroughly understanding it. 
