PLATYHELMINTHES TREMATODA 



or two other forms, however, unpaired. The vagina receives the 

 penis of another individual during copulation (Fig. 26),and does not 

 appear to have an homologue in the liver-fluke or other Digenea. 

 Life-Histories of the Polystomatidae. 1 Polystomum integer- 

 rimum. After the mutual fertilisation of two individuals, the 

 eggs are laid in the water by the protrusion of the body of the 

 parent through the urinary aperture of the Frog. About 1000 

 eggs are laid in the spring at the rate of 100 a day for ten 

 days. After about six weeks, the larva ('3 mm. long) hatches 



FIG. 25. Eggs of Monogenea. A, Eggs of Encotylabe pagdli v. Ben. -Hesse ; B, eggs 

 of Udonella pollachii v. Ben.-Hesse (with young forms just hatching out) ; C, egg 

 of Microcotyle labracis v. Ben.-Hesse. (After van Beneden and Hesse.) x 50. 



out, and swims about freely by means of bands of large ciliated 

 cells (Fig. 26, A) ; but if it does not meet with a tadpole within 

 twenty-four hours, it dies. Should it, however, encounter one, 

 the larva creeps along it in a looping fashion until it approaches 

 the opercular spout, or opening of the branchial chamber, on the 

 left side; into this it darts suddenly, fixes itself, and throws off 

 its cilia. Here it remains eight or ten weeks, feeding, in- 

 creasing in size, and forming the suckers from behind forwards. 



1 Zeller, Zeitschr. f. wiss. Zool. xxii. 1872, pp. 1, 168 ; also Bd. xxvii. 1876, p. 

 238 ; xlvi. 1888, p. 233. 



