232 PLATYHELMINTHES. 



and of water birds ; it can, however, attain maturity in the intestine 

 of the Dog, Mouse, and Eat. 



If we summarise the development of the endoparasitic Trematodes 

 we get the following results : 



(1) Development with a non-ciliated larva, which changes direct 

 into the sexual animal. No intermediate host, but transference to 

 another individual. Aspidogast-er and its allies. 



(2) Development with ciliated larva, which enters other animals 

 (Molluscs, Hirudinea, Fishes, Amphibians, Mammals), there encysts, and 

 takes on a second larval form (metastatic Trematodes). Holostomidae. 



(3) Development with ciliated or non-ciliated larva (Miracidium), 

 which passes actively or passively into another animal (Mollusc), 

 in which it becomes transformed into an asexual form (sporocyst 

 or redia), which produces Cercariae either directly, or after the 

 formation by internal budding and rarely by fission of more asexual 

 forms. These are the larvae of the adult sexual animal, and they 

 usually migrate into another host (Mollusc, or Crustacean, Insect, 

 Fish, Amphibian, and even Mammal), where they lose their larval 

 organs and encyst. Here they remain until they are passively 

 transferred into the alimentary canal of their permanent host 

 (almost always a Vertebrate), in which they become sexually mature 

 (digenetic Trematodes). Most Malaeotylea. 



The life-cycle of the digenetic Trematoda must be looked upon as an alterna- 

 tion of generations of the variety called heterogamy. That is to say, the 

 alternation is between a sexual generation (the so-called adult) and one or more 

 parthenogenetically producing generations (sporocyst, redia). The Mimcidium 

 is a larva which, becoming parasitic, loses its gut, nervous system, etc., and 

 degenerates into a sporocyst, or more rarely a redia. The sporocyst or redia 

 possesses a kind of diffuse ovary in the germ-cells, which develop either into 

 rediae or Cercariae (see above, p. 229). The redia is very similar to the sporocyst, 

 but is not so degenerate. The Cercaria is simply the larva of the final stage ; 

 it possesses special larval organs, e.g. tail and spine, which however may be 

 absent in cases in which they are not required ; e.g. Macrostomum, in which the 

 migration of the Cercaria into its final host is a passive one. 



On this view the germ-cells of the sporocysts and rediae are ova, and from 

 what is known of the development (see p. 229), they seem to be set apart at an 

 early stage (the end of cleavage). Some observers hold the view that we have 

 to do here with a kind of embryonic fission at a very early stage, the divided off 

 cells remaining latent for some time, and only developing later into sporocysts or 

 rediae. But whatever view be taken of the reproduction of the sporocysts and 

 rediae, there are two facts to be noted with regard to it ; (1) it starts, as does 

 sexual reproduction, from a one-celled stage, and (2) the phases of the develop- 

 ment of the so-called germ-cells, whether into rediae or Cercariae, seem closely 

 to resemble what is known of the development of the fertilised ovum into the 

 Miracidium. 



