FRESH-WATER PLANARIAN 



159 



The so-called excretory system (Fig. 72 B) does not justify the term 

 system. It consists in part of scattered flame cells (Fig. 73), which are 

 hollow and contain a mass of cilia extending into the cavity of the 

 cell. The cilia 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 epithelial 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 , 



Globules 

 of excretion 



Branches 

 of the cell 



If 



ifjk^ Excretory 

 I tubule 



Nucleus 



outside along the longitudinal 

 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. The scat- 

 tered flame cells do not justify 

 the name of tissue, though the 

 tubes do, but there are no struc- 

 tures which conform to the defini- 

 tion of organ. 



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 longi- 

 tudinal fibers which are separated by a set of obHque fibers. Bun- 

 dles of muscle fibers also pass through the body dorso vent rally, between 

 the branches of the intestine. 



The nervous system (Fig. 72 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 



A B 



Fig. 73. — Semidiagrammatic sketches to 

 illustrate the flame cells of a planarian. 

 (From Benham, in Larikester's, "A Treatise on 

 Zoology," by the courtesy of A. and C. Black.) 



A, part of the system of excretory tubes, 

 showing the relation of the flame cells to them. 

 Nuclei are seen in the walls and tufts of cilia 

 projecting into the tubes, adjacent to the nuclei. 



B, flame cell. 



