PLATYHELMINTHES 83 



large intestine which divides into two lobes, each branched into 

 ceca. The only external opening of the alimentary canal is through 

 the mouth. Excretory system and nervous system are well de- 

 veloped, while the reproductive organs are hermaphroditic. In the 

 Cestoda we find that an alimentary canal is absent, but that repro- 

 ductive, nervous, and excretory systems are well developed. 



Behavior. — The Turbellaria have well-developed tactile, olfac- 

 tory-gustatory and light percipient organs, but in the Trematodes 

 and Cestodes little development of these functions is found. In the 

 green marine worm, Convoluta roscoffensis^ geotropism has been 

 found to fluctuate with the rise and fall of the tides, even when the 

 animal was moved to an aquarium. Geotropism is dependent on the 

 statocyst. In Convoluta and in another Turbellarian, Vortex, it is 

 found that the parenchyma contains symbiotic unicellular green 

 algae (see page 26) similar in relations to that with the yellow cells 

 of Radiolaria. 



Regeneration. — Planaria are notable in their ability to regenerate 

 new parts, a single individual having been cut into one hundred 

 twenty transverse pieces, behind the eyes, and each piece regenerat- 

 ing a perfect worm. The tape worms are able to produce new pro- 

 glottids as long as the scolex remains. 



Fossil Relatives. — Fossil flatworms are rare. They occur from 

 the Pennsylvanian down to the present. 



Ancestry and Relationship to Other Phyla. — The Turbellaria 

 and the Ctenophora have possibly been derived from a common 

 ancestor, the bands of cilia in larval Turbellaria resembling some- 

 what the ciliary swimming plates found in Ctenophora. The 

 simplest Platyhelminthes are Turbellaria; then we come to the 

 Trematoda, in which the larval cercaria corresponds to the cysticerci 

 of the Cestoda. The form Ligula has been considered a connecting 

 link between the Trematoda and the Cestoda, since it has the 

 elongated body and multiple gonads of the tape worm, but repre- 

 sents only a single proglottid. 



Axial Gradient Theory of Child.— Dr. C. M. Child of the 

 University of Chicago, after experimenting with Planaria for many 

 years,^ elaborated an important theory of axiate organization, 

 according to which there is a gradient of metabolic activity in every 



8 Consult also Child, CM. 191 5. Individuality in Organisms; and Senescence 

 and Rejuvenescence. Published by Univ. of Chicago Press. 



