PHYLUM PLATYHKLMINTHES 



283 



of the intestine and develop into the adult Tccnia echinocomis 



— 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 feces of the Dog, may be taken into the digestive canal of 



Man or of one of the domestic animals, 



aiid 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- 



ccele 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 



undivided part behind the head, do not 



in turn give rise by budding to new pro- 

 \ glottides. Spontaneous transverse fission 

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



the regeneration of the lost portion. 



% 



Fio. 22(i. — Process of budding 

 in Microstomum <-., c'. 

 ciliated groove ; c. aye-spot ; 

 t. intestine ; m., »»', in..", >«.'" 

 mouth. (After Von Graff.) 



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. 



