vii PHYLUM TROCHELMINTHES 299 



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 modi- 

 fied trochosphere. The groups thus associated together may not 

 be genetically related : they may have become independently 

 developed from trochosphere-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 

 Dinopliilca and the Gastrotriclia. 



CLASS I.-ROTIFERA. 



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

 very abundant in pools, gutters, &c., 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 ccelome, nephridial tubes, gonads, a 

 nervous system, and sense-organs, and have therefore no real 

 relationship with the Protozoa. 



1. EXAMPLE OF THE CLASS Brachionus rul>cns. 



External Characters. Brachionus (Fig. 245) is one of the 

 commonest members of the class, being frequently found in 

 abundance in ponds, ditches, &c. It is about ^ mm. ( T J T 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, 

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

 the opposite or dorsal surface is convex both from before back- 

 wards, and from side to side. 



The anterior portion of the body projects from the lorica in the 

 form <>f a transverse disc(frv?.), with a prominent edge fringed with 



