Phyla Platyhelminthes and Nemertina 403 



definite sense organs developed, and among all there is concentration 

 of nervous tissue. There is a pair of cerebral ganglia which constitute 

 the "brain." From it, two or more longitudinal cords pass posteriorly ; 

 these are connected by cross connections and the whole system resembles 

 a ladder. This concentration of nervous tissue again represents a con- 

 siderable advance over that found in the coelenterates and ctenophores 

 where there was only a diffuse network of neurons. 



While there is no specialized system for either circulation or respira- 

 tion, the excretory system is well developed. It is of the protonephridial 

 type. In this, each individual protonephridium is a flame cell (Fig. 

 134,5). A flame cell consists of an elongated cell with a central cavity; 

 within this central cavity are long cilia which beat to create currents. 

 Water gathered by these cells is then passed into collecting tubes which 

 gradually combine to form larger ducts. These ducts open to the outside 

 through special pores or at times they open into a contractile bladder. 

 The materials are moved along the larger ducts by the currents created 

 by small flame cells recessed within the walls. From various experi- 

 ments, it appears that the chief function of this system is that of osmo- 

 regulation. This conclusion is based on several carefully done experi- 

 ments and also on the fact that this system is not so well developed in 

 the flatworms dwelling in marine and brackish water. Thus in its 

 activity the protonephridal system can probably be compared to the 

 contractile vacuole of the protozoans. 



The flatworms are nearly aways hermaphroditic, but cross fertiliza- 

 tion normally occurs. The gonads are formed from mesenchymal cells 

 and primitively consist of many scattered masses of cells. In more 

 advanced types, these masses are reduced to a few larger gonads ; com- 

 monly the testes remain scattered while the ovaries coalesce into one or 

 two larger masses. The ovary proper is separated from the yolk glands 

 so that the yolk cells produced are separate from the egg cells. A com- 

 plicated series of ducts has evolved within the phylum for carrying the 

 sex products. Usually there is a pair of oviducts and a pair of sperm 

 ducts. These may unite and pass out through a common genital pore 

 or may have separate openings. Well-developed copulatory apparatuses 

 often are present at the end of the sperm ducts. These may be protru- 

 sible penises or cuplike sacs provided with spines. 



Asexual reproduction is commonly employed. The free-living forms 

 are able to fragment or to divide by transverse fission. Remarkable 

 regeneration abilities make possible the success of this method. 



