INTESTINAL WORMS. 7 
Danish zoologist MUELLER classed amongst his Infusories minute 
worms with tails, to which he gave the generic name of Cercaria. 
The accurate observations of Nrrzscu taught us to define the genus 
more completely, and supplied a lively picture of the form and 
movements of a worm that to the naked eye seems like a moving 
point!. He compared these animals to Distomes that have had a 
tail-like appendage attached to them, and their motions to those of 
a Vibrio: each of these structures, the body and the tail, had a 
separate motion: when the body moved on by creeping and sucking 
the tail was at rest; and, conversely, when the tail undulated rapidly 
it forced along with it the body, which seemed now to have no 
independent motion of its own. Nurrzscu further saw that Cercarie 
cast their tail, and observed in Cercarta ephemera that it fixes 
itself, covers itself with a shell that presently hardens, and thus 
becomes, as it were, a pupa. Such pups remain unchanged for 
months: what was to succeed remained unknown. Afterwards 
similar observations were made by others, particularly by Von 
STEBOLD. STEENSTRUP® shewed that the pupe change into Di- 
stomes after the lapse of several months. Cercariw live as parasites 
within the bodies of different Molluscs, as Zymneus and Planorbis. 
But this does not terminate the surprising series of these changes. 
Cercarie, the larvee of Distomes, do not arise immediately from the 
egos of these last. Within the above-named Molluses, and in some 
others, little bags of an oblong form (germ-pouches) are found, in 
which a peculiar organisation and motion may sometimes be traced, 
but which in other species are motionless, and contain Cercari@ in 
a more or less advanced state of development. Considered as para- 
sites of the germ-pouches, these have been named parasites of the 
second order: yet they are not parasites, but the progeny of these 
vermiform germ-pouches’. How these last originate from the young 
of Distomes has not been absolutely ascertained. The young animals, 
1 C. L. Nirzscn Beitrag zur Infusorienkunde, Halle, 1817, 8vo. 
* SreensrruP Alternation of Generation. Translated by Busk for Ray Soc. 
3 BosaNnus, who discovered such vermiform germ-sacs of a yellow colour in Lim- 
neus stagnalis, named them King’s yellow worms; Isis, 1818, s. 729. The celebrated 
V. Baer published many similar observations in Nov. Act. Ces. L. C. Nat. Cur. 
Vol. xu. P. 2, pp. 605—659, Taf. xxxt.; as also the far-famed investigator of the 
lower animal forms V. S1mBoip in Burpacu’s Physiol. 2" Ausgabe, s. 186, &e. 
