vii PHYLUM TROCHELMTNTHES 323 



behind the pre-onil circlet, leads into an alimentary canal, which at 

 first runs nearly transversely, and then bends round so as to extend 

 back towards the narrow end, near which it opens on the exterior 

 by an anal aperture. About the middle of the broader (anterior) 

 end of the trochophore is a thickening, the apical plate (SP.), pro- 

 jecting from which are usually a 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, >vhich is the nerve-centre 

 of the larva. A pair of ciliated 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- 

 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 in- 

 cluded in the same phylum, are the Dinophilea and Histriobdelka. 



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 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. 264) is one of the 

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

 dance in ponds, ditches, &c. The female is about £ mm. ( T Vin.)in 



Y 2 



