PELAGIC ZONE— SURFACE LIFE 



453 



of these curious Worms, like those of the last-named group, are 

 well represented in the surface waters of the sea, and some adult 

 Nemertines have given up creeping and taken to a pelagic life. 



One of the most remarkable {Pela- 



gonemertes) is represented in fig. 



1308. 



Piankion Thread- Worms {Ne- 



mathehnia). — Among the com- 



Fig. 1308. — Pelagic Nemertine Worm 

 [Pelagoneinertes], reduced 



Fig. 1309. — Night-LIglit Animalcules 

 [Nociiluca], enlarged 



monest inhabitants of the surface waters are the curious little 

 fish-shaped Arrow- Worms {Sagitia, Spadella, &c.), which con- 

 stitute a special group {Chcetogjiaiha) that is generally supposed 

 to be an outlying constituency of the Thread- Worms (see vol. iii, 

 p. 21), though its affinities are doubtful. 



Fig. 1310. — Shells of Ray-Animalcules {Radiola?ia), imbedded in the 

 fibrous skeleton of a sponge 



Plankton Zoophytes {Caelenterata). — A bewildering variety of 

 jelly-fishes, belonging to all sorts of groups, abound in the surface 

 waters. Many of them have abandoned altogether the fixed 

 zoophyte-stage that is found in the life-history of many such 

 creatures (see vol. iii, p. 349). Perhaps the most interesting 



Vol. IV. 



124 



