5Q 



NATURAL HISTORY. 



of generations here as in the hydroids : the adult worm produces eggs, 

 each of which produces a single embryo ; each embryo gives rise to several 

 rediee, each redia to several cercarise, while each cercaria makes an adult 

 worm. The sexual generation alternates with two asexual buddings. 

 Occasionally a redia will produce several generations of redia3, thus still 

 further complicating the history, while a sporocyst may divide into several 

 sporocysts before the formation of redige begins. 



Flukes are comparatively common, and the molluscs are fearfully sub- 

 ject to their attacks. Sometimes three or four different kinds of cercaria} 

 will occur in a single snail. Fishes and frogs are often hosts of the mature 

 fluke, and without doubt the parasites enter their bodies from the snails 

 which they eat. Even man himself is not exempt from their attacks, five 

 distinct species of fluke being enumerated as being human parasites. Mr. 

 Thomas suggests that the liver-fluke may obtain entrance to the human 

 system along with water-cress eaten as a relish. 



The animal shown in the next illustration is, possibly, one of the great- 

 est wonders of nature, for it is a double animal. It is not one of those 



queer and unexplained freaks like the 

 Siamese twins, which startle us because 

 they are so rare. Monstrosities occur 

 in the worms as well as in the human 

 species, but this is really a regularly 

 occurring phase in the history of a 

 worm allied to the flukes. Di-plozoon 

 (the name means double animal) was 

 found by Nordmann, a German profes- 

 sor, in 1831, and in more recent years 

 its life history has been studied by sev- 

 eral observers. It occurs as a parasite in the gills of the bream, and con- 

 sists of two individuals, each a little less than half an inch in length, joined 

 together in the form of a cross, the left tail belonging to the right head 

 and vice versa. The two are not always united ; when young they exist 

 separately ; but when they meet on the gills of a fish one applies his 

 sucker to a projection growing on the back of the other, and thus they 

 hold together. Gradually the union becomes more intimate, until they 

 grow together and literally as well as metaphorically the twain become 

 one flesh. 



Last of the. flat worms come the tape-worms, a group usually regarded 

 with unpleasant feelings, though their habits render them both important 

 and interesting. All are parasitic and affect man and other animals. Of 

 these there are many, far too many, forms ; some are comparatively sim- 



Fig. 56. — Diplozoon enlarged. The black, branch 

 ing portion is the digestive tract. 



