182 



PRINCIPLES OF ANIMAL BIOLOGY 



CR- 



reproduce by means of parthenogenetic ova. The liver fluke, Fasciola 

 hepatica, is an excellent example whose life history has been thoroughly 

 studied. This life history, illustrated in Fig. 140, is essentially as follows. 

 From the fertilized egg produced by the adult emerges a ciliated larva, 

 the miracidium (Fig. 140, a, b), which must live in water. The mira- 

 cidium finds a snail of the genus Lymnsea, bores its way into the snail 

 and develops into a bag-like individual, the sporocyst. The sporocyst 

 (c) produces within its cavity a number of parthenogenetic ova which 



develop into individuals very different 

 from the sporocyst. These are redicc. 

 The redise {d) are provided with a short 

 digestive canal, some ear-like appendages 

 and a birth pore. They emerge from the 

 sporocyst but remain within the snail. 

 The redise produce parthenogenetic ova 

 which may develop into more rediae or 

 into a tailed form, the cercaria. In the 

 event that further generations of rediae 

 are produced by partheno- 

 genetic means, cercariae are 

 nevertheless finally produced. 

 The cercaria (e) is the larval 

 form from which by growth 

 and certain changes the adult 

 liver fluke (g) arises. In this 

 species there are thus always 

 two or more generations of a 

 larval type which produce 

 parthenogenetic ova and only 

 one generation producing 

 fertilized ova. The origin of 

 parthenogenetic ova in the 

 redia of another species and 

 a much magnified ovum which 

 has undergone maturation are 

 shown in Fig. 141, A and B. 

 The polar bodies are plainly indicated and give evidence that these cells 

 are true ova. 



In the succeeding instances psedogenesis of the bisexual type occurs. 

 The ctenophore Bolina hydatina while still in the larval condition becomes 

 sexually mature and its gonads (reproductive organs) produce eggs or 

 sperms. Fertilized eggs develop in the usual manner. The larva even- 

 tually assumes the adult condition and the gonads, which had degen- 

 erated, again attain sexual maturity and produce eggs and sperms. 



A B 



Fig. 139. — Psedogenesis in the fly Miastor. A, 

 sagittal section (greatly enlarged) through an 

 embryo of Miastor americana, showing four oogonia 

 (oogS) ; B, mother larva of Miastor with young psed- 

 ogenetic larvae. (A from Hegner's Germ Cell Cycle, 

 MacmiUan Co.; B from Folsom after Pagenstecher.) 



