GENERAL ZOOLOGY 



Nucleus 



"Flame' 



Flame 

 bulbs 



Nucleus 



Fig. 11.3. Protonephridia of turbellarians. A, individual flame cell of a planarian. B, 

 group of flame bulbs and their canals, with a single nucleus; from the rhabdocoel Mesostoma. 

 (Redrawn, after various authors, from E. Bresslau in W. Kijkenthal and T. Krumbach, 

 1933, Handbuch der ^oologie.) 



larger tubes, which open to the exterior by a number of dorsolateral excretory 

 pores. The flame bulbs, or flame cells, are so named because of the flickering 

 tufts of cilia beating within their cavities (Fig. 11.3). These cavities are 

 intracellular spaces and are continued as intracellular ducts within the cells 

 making up the tubules. A system composed of these and other similar units is 

 a distinctive feature of trematodes and cestodes, as well as of turbellarians; 

 such systems also occur in animals belonging to other phyla. Flame bulbs are 

 termed protonephridia, and any svstem of flame bulbs and ducts leading to 

 external orifices is a protonephridial system. In ascribing an excretory function 

 to protonephridia, it is supposed that the tufts of cilia in the flame bulbs, 

 and other cilia lining the tubules, set up currents flowing toward the external 

 openings. Fluids, containing metabolic wastes in solution, may presumably 

 enter the system from the lymph by passing through the flame cells, and may 

 be driven from the body through the excretory pores. It is probable, how- 

 ever, that the chief function of the protonephridia, as of protozoan contractile 

 vacuoles, is in maintaining water balance, with excretion as an incidental 

 process. 



The Nervous System and Responsiveness. The nervous system is well 

 developed in correlation with the complex musculature of planarians. A pair 

 of eyes, consisting of cup-shaped groups of light-sensitive cells, is found in 

 the dorsal surface of the head region (Fig. 11.4). Beneath the eyes lies a 

 concentration of nervous tissue, the brain or cerebral ganglia. Two nerve cords 

 extend to the posterior end of the body, and there are transverse connections 

 between these cords throughout their length. These elements altogether con- 

 stitute what may be termed a central nervous system (Fig. 11.5). From the 



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