DEVELOPMENT OF ENTOZOA AND ANNELIDA. 569 



succession of these changes. Thus between every act of 

 generation there intervene two sets of gemmations, by which 

 a single embryo may produce a multitude of Cercarise ; and 

 the conversion of the Cercaria into the Distoma involves, in 

 addition, a metamorphosis not less complete than that of 

 Insects. The body (fig. 310) within which the Cercariae are 

 developed, and which is the second remove from the Dis- 

 toma, has been called their "nurse ;" and that (fig. 309) from 

 which the nurses themselves are developed, and which is the 

 first remove from the Distoma, has been called their " grand- 

 nurse." 



744. Among the Annelida, or Worms properly so called, 

 there is considerable variety in the history of development ; 

 some of them, as the Leech and Earth-worm, coming forth 

 from the egg in a nearly perfect state ; whilst in most of the 

 marine worms that state is not attained until long after the 

 embryo has begun to lead an independent life. This embryo, 

 on its first emersion from the egg, very commonly has an oval 

 or roundish body, furnished with one or more bands of cilia, 

 by the agency of which it swims freely in the water ; the 

 body then gradually becomes elongated, and additional bands 

 of cilia make their appearance ; and after a time a mouth and 

 intestinal canal are formed, indications of eyes and of a seg- 

 mental division show themselves, and the cilia disappear, their 

 place being usually taken by bristly appendages. The body is 

 gradually elongated by the production of additional segments, 

 sometimes to the number of several hundred ; each new seg- 

 ment being formed between those which were previously the 

 last and the last but one. Thus the formation of the body of 

 the Worm is really accomplished by a process of continuous 

 gemmation ; and it is therefore the less surprising that some 

 of these worms should be capable of producing independent 

 buds ( 727), which buds, however, not unfrequently resem- 

 ble the segments of the Tape-worm, in containing nothing 

 else than the generative apparatus, save locomotive organs for 

 the purpose of dispersing its products. 



745. It was in the class of INSECTS that the phenomena of 

 metamorphosis were first studied ; and notwithstanding the 

 familiarity of the leading facts of the case, it is desirable to 

 recapitulate them here, for the sake of showing their relation- 

 ship to those we have been already considering. There are 



