SECT, vn PHYLUM TROCHELMINTHES 317 



number of sensory cilia (WS.) ; and in many trochophores eye-spots 

 and a pair of short tentacles occur in close relation with the apical 

 plate, which is the nerve centre of the larva. A pair of tubes 

 the excretory organs or nephridia (Neph.) may be present. 



In the higher groups in which this form of larva occurs, the 

 adult condition is attained by modifications and new developments 

 of so radical a nature that the transition from larva to adult is of 

 the nature of a metamorphosis. Sometimes the narrow part of the 

 larva elongates and becomes divided into a series of sections fore- 

 shadowing the metameres of the adult animal ; in other cases, in 

 which no metamerism occurs, radical changes of other kinds lead 

 to the adult form. But in all these higher groups, whatever the 

 nature of the changes involved, there is a metamorphosis, and the 

 adult animal is totally unlike the larva. In a small number of 

 forms now to be dealt with, however, there is no such radical 

 change, and the adult may be looked upon as a somewhat modified 

 trochophore. The groups thus associated together may not be 

 genetically related : they may have become independently 

 developed from trochophore-like ancestors, but the possession of 

 the general characters which have been referred to above renders 

 it convenient to group them together and regard them as con- 

 stituting a small but well-marked phylum. The groups referred 

 to are the Rotifera or Wheel-animalcules, together with the 

 Gastrotricha. Associated with these, though scarcely to be included 

 in the same phylum, are the Dinophilea and Histriobdellea. 



CLASS L-ROTIFERA. 



The Rotifers or " Wheel-animalcules " are microscopic creatures, 

 very abundant in pools, gutters, etc., and formerly classed with the 

 Infusoria, to which several of them bear a superficial resemblance. 

 But in spite of their minute size they are multicellular animals, 

 having an enteric canal, a spacious body-cavity, nephridial tubes, 

 gonads, a nervous system, and sense organs, and have therefore no 

 real relationship with the Protozoa. 



1. EXAMPLE OF THE CLASS Brachionus rubens. 



External Characters. Brachionus (Fig. 269) is one of the 

 commonest members of the class, being frequently found in abun- 

 dance hi ponds, ditches, etc. The female is about ^ mm. ( T V in.) in 

 length, and is divisible into two distinct parts a broad anterior 

 region, the trunk, and a slender movable tail (t.). The trunk is 

 enclosed in a glassy cuirass or lorica (lr.), formed by a thickening 

 of the cuticle and produced into several spines : the tail is wrinkled 

 superficially and ends in two slender processes, together forming 

 a kind of forceps. One surface of the trunk is flattened and, 

 owing to the position of the mouth, is considered as ventral ; the 



