LIFE HISTORY OF FLUKE-WORMS. 107 
about in its place and secreting a slime, a cyst is gradually 
formed, with aspherical shell. This constitutes the ‘‘ pupa’”’ 
state of the Cercaria. Steenstrup thinks that the Cercaria 
casts athinskin. In this state the body can be seen through 
the shell of the cyst, as in Fig. 68, C, where the circle of 
spines embedded around the mouth is seen. The encysted 
Cercarie remain in this state from July and August until 
the following spring ; and during the winter months, in 
snails kept in warm rooms, they change into Distomas (Fig. 
68, D), the mature fluke differing, however, in some im- 
portant respects from the tailless larve. In nature they 
remain from two to nine months in the encysted state. 
“* Now,’ asks Steenstrup, ‘‘ whence come the Cercarie ?”’ 
Bojanus states that he saw this species swarming out from the 
‘‘ king’s yellow worms,’ which are about two lines long and 
occur in great numbers in the interior of snails. From these 
are developed the larval Distomes, and Steenstrup calls them 
the ‘‘ nurses ’’ of the Cercariz and Distomes. They exactly 
resemble the ‘‘ parent-nurses’’ (Fig. 68, A, and 70), and, 
like them, the cavity of the body is filled with young, which 
develop from egg-like balls of cells. Steenstrup was forced 
to conclude that these nurses originated from the first nurses 
(Fig. 68), which he therefore calls ‘‘ parent-nurses.’’ Here 
the direct observations of Steenstrup 
on the Cercaria echinata came to an 
end, but he believed that the parent- 
nurses came from eggs. The link-in 
the cycle of generations he supplied 
from the observations of Siebold, 
who saw a Cercaria-like young (Fig. 
69, B) expelled from the body of the 
ciliated larva of Monostomum muta- 
bile. Steenstrup remarks that “‘ the ee 
first form of this embryo is not un- Fig. 69.—Development of 
like that of the common ciliated pro- {ongstomum. A. ciltated larva; 
geny of the Trematoda, as they have %: nuee—After Siebold, 
been known to us in many species for a long time, and it 
might at first sight be taken for one of the polygastric in- 
fusoria of Ehrenberg, which also move by cilia ; whilst in 
