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 Cercarim are 
developed, and which is the second remove from the Dis¬ 
toma, has been called their “ nurseand 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 
