. Zoology for high schools and colleges. Zoology. LIFE HISTORY OF FLUKE-WORMS. 149 about in its place and secreting a slime, a cyst is gradually formed, with a spherical shell. This constitutes the " pupa " state of the Cercaria. Steenstrup thinks that the Cercaria casts a thin skin. In this state the body can bo seen through the shell of the cyst, as in Fig. 98, 0, where the circle of spines embedded around the mouth is seen. The encysted Cercarise remain in this state from July and August until tbe following spring ; and during the winter months, in snails kept in warm rooms, they
. Zoology for high schools and colleges. Zoology. LIFE HISTORY OF FLUKE-WORMS. 149 about in its place and secreting a slime, a cyst is gradually formed, with a spherical shell. This constitutes the " pupa " state of the Cercaria. Steenstrup thinks that the Cercaria casts a thin skin. In this state the body can bo seen through the shell of the cyst, as in Fig. 98, 0, where the circle of spines embedded around the mouth is seen. The encysted Cercarise remain in this state from July and August until tbe following spring ; and during the winter months, in snails kept in warm rooms, they change into Distomas (Fig. 98, D), the mature flake differing, however, in some im- portant respects from the tailless larvae. In nature they remain from two to nine months in the encysted state. " Now," asks Steenstrup, " whence come the Cercarise ?" 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 Cercarise and Distomes. They exactly resemble the " parent-nurses " (Fig- 98, A, and 100), 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. 98), which he therefore cafls " ; 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. 99, B) expelled from the body of the ciliated larva of Monostomum muta- bile. Steenstrup remarks that " the first form of this embryo is hot un- Fig. of like that of the common Clhatea pro- m, mouth ; b. eyes
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