No.417.] WORTH-AMERICAN INVERTEBRATES. 747 
or absence of appendages on the body; and mode of life, — 
whether fixed or free-swimming. 
The Rotatoria are a cosmopolitan group, so that the same 
species may be, and as a matter of fact frequently are, found 
in America, Europe, India, China, and Australia. Our rotifer 
fauna is, on the whole, so far as known, almost identical with 
that of the one other well-known continent, — Europe. From 
this it results that the only entirely satisfactory key for 
America or any other country would be one including all 
the species of the group. Two hundred and forty species have 
thus far been recorded from America; it is probable that 
nearly as many more will be found before the list can be con- 
sidered to approach completeness. Under these circumstances 
the following key to known American species can be considered 
to have merely provisional value. Moreover, the species of 
many of the larger genera can be determined only from full 
descriptions and detailed figures; the key should in such cases 
be used in connection with fuller accounts. 
The classification of the Rotatoria is in an unsatisfactory 
condition. The system employed by Hudson and Gosse in 
their monograph of the Rotifera is used almost exclusively, 
and is therefore adopted in essentials in the present key. 
But it is undoubtedly unsatisfactory in many ways, separating 
widely many closely related species, and bringing together 
some that are widely divergent in essential structure. A 
better classification has been outlined by Wesenberg-Lund, but 
this has not yet been worked out in sufficient detail to make 
its use practically satisfactory in such a key. 
Owing to the large number of species of the Rotatoria and 
the frequent difficulty of assigning definite distinguishing 
Characteristics even to the genera, it will be necessary for the 
key to the genera and species to be purely artificial in char- 
acter. I give first, therefore, a systematic synopsis of the 
orders, suborders, and families. This synopsis is based on 
that given by Hudson and Gosse in the monograph of the 
Rotifera. I have introduced certain divisions not given by 
Hudson and Gosse, — such as the general division into Mono- 
£ononta and Digononta ; this division is one which is employed 
