FRESH-WATER PLANARIAN 



161 



The excretory system (Fig. 77 B) consists in part of scattered flame 

 cells (Fig. 78), which are hollow and contain a mass of cilia extending into 

 the cavity of the cell. The ciHa by their movements suggest a waving 

 flame. The cavities of the flame cells communicate directly with slender 

 tubes. The walls of these tubes are composed of a single layer of epi- 

 thelial cells or, in the opinion of some observers, the tubes are made up of 

 tubular cells placed end to end. These tubes lead into larger and larger 

 ones until finally a pair of longitudinal and much coiled tubes is reached. 

 One of these coiled tubes lies on each side of the body and the two are 

 connected with a transverse tube at the anterior end, opening by two 

 small pores on the dorsal surface behind the eyespots. Other openings 

 to the outside along the longi- 



tudinal tubes also exist. These 

 tubes are full of fluid containing 

 the waste matter, and the flame 

 cells, by means of their cilia, 

 produce a current which carries 

 this fluid outward. 



Muscle tissue is present in 

 the form of sheets and bundles, 

 the fibers of which run in differ- 

 ent directions. An outer cir- 

 cular layer lies just under the 

 epidermis, and below it are 

 layers of external and internal 

 longitudinal fibers which are 



Nucleus 



Globules 

 of excretion 



^, Flame 

 f cells 



Branches 

 of the cell 



Excretory 

 tubule 



B 



Fig. 78. — Semidiagrammatic sketches to 

 illustrate the flame cells of a planarian. {From 

 Benham, in Lankester's, " A Treatise on Zoology," 

 ,1 , f -I y by the courtesy of A. and C. Black.) A, part 



separated by a set OI oblique ^j ^^le system of excretory tubes, showing 

 Bundles of muscle fibers the relation of the flame cells to them. Nuclei 



are seen in the walls and tufts of cilia project- 

 ing into the tubes, adjacent to the nuclei. 



fibers. 



also pass through the body dorso- 



ventrally, between the branches B, flame cell. 



of the intestine. 



The nervous system (Fig. 77 C) is very simple. Two masses of 

 nervous tissue containing nerve cells and fibers lie below the eyespots 

 and form a nerve center. Such a collection of nerve cells is termed a 

 ganglion. From these ganglia cords of nervous tissue, which are gang- 

 lionic in character, pass back, one on each side of the body. Both the 

 ganglia and these two cords are connected by commissures. A com- 

 missure is a bundle of nerve fibers which in a bilaterally symmetrical 

 animal crosses the middle plane of the body and connects corresponding 

 nerve centers on opposite sides. A similar connection between centers 

 on the same side is known as a connective. A nerve is a bundle of nerve 

 fibers not in a center which conveys impulses from one part of the body 

 to another. From both ganglia and nerve cords, peripheral nerves are 



