PHYLUM PLATYHELMINTHES 



263 



Asexual reproduction also occurs in some Platyhelminthes. 

 In some Rhabdocosle Turbellaria (Microst&mum) a process of bud- 

 ding (Fig. 212) results in the formation of strings of sexual indivi- 

 duals 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 men- 

 tioned, 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 zooids of a colony. There is 

 this essential difference between this 

 formation of proglottides and the asexual 

 multiplication by budding in Micro- 

 stomum, that in the former the proglot- 

 tides, when they have been formed by 

 segmentation of the undivided part be- 

 hind the head, do not in turn give rise by 

 budding to new proglottides. Spontane- 

 ous transverse fission has been observed 

 in certain Tricladicla, and has often been 

 observed to be followed by the regenera- 

 tion of the lost portion. 



m 



m 



7TL 



6. DISTRIBUTION, MODE OF OCCURRENCE, 

 AND MUTUAL RELATIONSHIPS. 



Of all the great groups of the animal 

 kingdom above the Protozoa the Platy- 

 helminthes are the widest in their distri- 

 bution. 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 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. 



As regards their mode of life, they present almost every possible 

 gradation between free-living forms which procure their food, con- 

 sisting of minute animals and plants, by their own exertions, and 

 forms that are only capable of living in a special part of the 

 interior of a certain other animal, and are quite incapable of pro- 

 curing food for themselves, living by the passive absorption of the 

 juices of their host or of its digested food. The Turbellaria are 

 for the most part free living, and their food consists of small 



FIG. 212. Process of budding 

 in IVIicrostomum <:, c.' 



ciliated groove ; e. eye-spot ; 

 i. intestine ; m., >u.', m.", m."' 

 mouth. '(After Von Graff.) 



