CHAPTER IV. 



PLATHELMINTHES. 



I. TURBELLARIA. 



Systematic : A. Dendroccelid^, with branched intestine. 



(a) Polycladida, with a median chief 

 intestine, which gives off numerous 

 branches. 



(b) Tricladida, without chief intes- 



tine ; three intestinal branches 

 are directly attached to the 

 pharynx. 

 B. Rhabdoccelidj:, with straight unbranched 

 intestine or w^ithout intestine. 



(a) Rhabdocoela, with a spacious cavity 



in the region of the intestine. 



(b) Alloiocosla, the cavity in the in- 



testinal region reduced by the 

 great development of the paren- 

 chymatous tissue. 



(c) Acoela, without distinct intestine. 



The Turbellaria which inhabit the land and fresh water 

 (Tricladida and Bhabdoccela) as well as many marine forms 

 {Polycladida) have a direct development, whereas other Poly- 

 clads undergo a metamorphosis in which there is a free- 

 swimming ciliated larva. The development best known is 

 that of the Poljclads, and of these we will first consider the 

 forms which develop directly. Closely related to these 

 Polyclads are those with a metamorphosis, for in the latter 

 the embryonic development proceeds in much the same way 

 as in the former. The embryonic development of the 

 Triclads, on the contrary, is different, while that of the 



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