PHYLUM PLATYHELMINTHES 



283 



of the intestine and develop into the adult Tcenia echinococcns 

 which are very small as compared with the size of the cyst and 

 as compared with other tape-worms. The eggs, passing out with 

 the f^ces of the Dog, may be taken into the digestive canal of 

 Man or of one of the domestic animals, 

 and the minute embryos escaping, reach 

 some organ, such as the liver or lung, in 

 which they are capable of developing into , 

 a comparatively enormous cyst. 



Asexual reproduction also occurs in 

 some Platyhelminthes. In some Rhabdo- 

 coele Turbellaria (Microstomum) a process 

 of budding (Fig. 226) results in the forma- 

 tion of strings of sexual individuals which 

 may eventually separate ; the new bud is 

 always formed from the posterior end of 

 the last individual of the string. 



The sporocyst stage in the Trematodes 

 may, as already mentioned, multiply by 

 budding or fission. The formation of new 

 proglottides in the Tape-worm may be 

 looked upon either simply as growth ac- 

 companied by segmentation, or as asexual 

 multiplication, according as we regard the 

 proglottides as segments of a simple animal 

 or as zooids of a colony. There is this 

 essential difference between the formation 

 of proglottides and the asexual multipli- 

 cation by budding in Microstomum, that in 

 the former case the proglottides, when they 

 have been formed by segmentation of the Fltt - 226. Process of budding 



i . . -. ,11-1,1 i 11 in Microstomtun. c., c'. 



undivided part behind the head, do not ciliated groove ; . eye-spot .; 



in turn give rise by budding to new pro- m^?. tin ( lftS voA cnlff!)''' 

 glottides. Spontaneous transverse fission 



has been observed in certain Tricladida, and is often followed by 

 the regeneration of the lost portion. 



6. DISTRIBUTION, MODE OF OCCURRENCE, AND MUTUAL 

 RELATIONSHIPS. 



Of all the great groups of the animal kingdom above the 

 Protozoa the Platyhelminthes are the widest in their distribution. 

 Members of the phylum occur on land, in fresh-water down to the 

 bottom of some of the deepest lakes, on the sea-shore, in the deep 

 sea, and on the surface of the ocean ; and parasitic Flat-worms live, 

 in one phase or another, in animals of nearly every class of the 

 Metazoa. 



