350 C. T. HUDSON. 



though unfinished sketch in vol i and ii of the ' Popular Scien- 

 tific Review ' (1862 and 1863) ; and finally I have adhered to 

 the utmost to the old nomenclature^ and endeavoured to meddle 

 as little as possible with the great Prussian naturalist's original 

 plan. 



The class Rotifera falls then, I think, into four natural orders 

 according to their modes of locomotion. There are some that 

 swim only; others that both swim and creep like a leech; those 

 that both swim and skip ; and lastly,, those that when adult are 

 fixed : and these orders differ in the main from each other in 

 the form and use of the foot. 



In each order too there are typical genera, round which the 

 rest may be grouped, differing from each other in the shape of 

 the trochal disc, and the position of its ciliary wreaths, as also 

 in the structure of the manducatory organs ; and sometimes in 

 other important points as well. 



But before I describe the families that can be formed round 

 these typical genera, I must digress a little to explain certain 

 technical names which I shall find it necessary to use. 



Gosse^s exhaustive treatise on ' The Manducatory Organs 

 in the Class Rotifera,' enables us to see that these organs 

 present seven or eight types of structure, distinguished from 

 each other by the prominence of some particular part. 



To make this clear it may be as well to state that in the 

 typical mastax of a Brachionus there are two hammer-like 

 bodies (mallei) (fig. 14, B ; c, d), which work on a kind of split 

 anvil (incus) (fig. 14, B, ef) ; and that each malleus consists of 

 an upper part or head (uncus) (fig. 14, B, d), and a lower or 

 handle (manubrium) (fig. 14, jB, c) ; while the incus also con- 

 sists of two, the upper divided into two symmetrical halves 

 (rami) (fig. 14, B, e), which are supported on the lower piece 

 (fulcrum) (fig. 14, B,f) ; these hard portions of the mastax 

 are termed the trophi. 



Now, in Brachionus (fig. 7) all the trophi are well developed; 

 but the other typical manducatory organs may be arranged in 

 a series in which the mallei are successively degraded, while 

 continually greater prominence is given to the incus ; at least 



