PHYLUM PLATYHELMINTHES 167 



systems of organs is correlated with the thickness of the body- 

 wall. The excretory system is necessary, since it is no longer 

 possible for the animal to get rid of the waste products of metabo- 

 lism through the general surface of the body. Likewise a system 

 of ducts is required to transport the germ-cells to the exterior. 

 No circulatory system appears in the flatworms, but in most cases 

 the food is transported directly to the tissues through the much- 

 branched digestive tract, which serves, as in the CCELENTERATA 

 and CTENOPHORA, as a gastro vascular cavity. 



Definite bilateral symmetry is exhibited by flatworms and 

 should be considered an advance in morphological development, 

 since the most successful animals have their bodies constructed 

 on this plan. With bilateral symmetry is probably correlated 

 the concentration of nervous tissue, the brain, in the head; the 

 end of the body directed forward in moving would receive sen- 

 sations first, and nerve-cells would be developed in the region of 

 greatest stimulation. It is believed by some authorities that 

 the body-cavity in the larval stages (sporocyst and redia) of 

 liver-flukes represents the ccelom (p. 89) and that the reproduc- 

 tive ducts of the adults should be considered true ccelomic 

 cavities. 



Our present knowledge of the flatworms seems to indicate that 

 they, as well as the CTENOPHORA, have evolved from ccelenterate 

 stock. Forms like the simplest TURBELLARIA, the RHADOCCE- 

 LIDA, have probably given rise to the more complex members 

 of that class. From these also were probably derived the 

 TREMATODA, no doubt in response to the changed conditions of 

 life resulting from a parasitic habit. Many of the adult CES- 

 TODA appear so closely related to certain TREMATODA that these 

 two classes may have arisen together, or else the former have 

 become separated from the complex TREMATODA (Digenea) as 

 a distinct group. 



Some authorities believe that the two curious animals Cteno- 

 plana and Cceloplana are connecting links between the CTENO- 

 PHORA and PLATYHELMINTHES. Ctenoplana has been recorded 



