136 



COLLEGE ZOOLOGY 



Nucleus 

 Cytoplasm 



Cell lumen 

 Cilia 



Flame cell 



Excretory duct 



Excretory 

 tubule 



Figure 69. Excretory system of a fresh-water planarian. On the right is shown a single flame 

 cell attached to a portion of an excretory duct. Arrows in flame cell indicate direction of flow 

 of materials. {Left after L.H. Hyman, The American Biology Teacher, 1956.) 



Circulation of the digested food is accom- 

 plished within the branches of the digestive 

 system and in the fluid-filled spaces in the 

 parenchyma. 



The excretory system consists of a com- 

 plex network of small tubes on each side, 

 from which flame cells branch (Figs, 68 and 

 69). The flame cell (Fig. 69) is large and 

 hollow, with a group of flickering cilia ex- 

 tending into the central cavity, which create 

 a current and force the collected fluid 

 through the tubules which open on the sur- 

 face by several minute pores. The muscular 

 system consists principally of three sets of 

 muscles, a circular layer just beneath the 

 epidermis, then a longitudinal layer imme- 

 diately below the circular muscle cells, and 

 dorsoventral muscles lying in the paren- 

 chyma (Fig. 70). 



There is a well-developed nervous system 

 (Fig. 68), consisting of an inverted V- 

 shaped mass of tissue, the brain, and two 

 ventral longitudinal nerve cords connected 

 by transverse nerves. From the brain, nerves 

 pass to various parts of the anterior end of 



the body. The highly pigmented eyes are 

 sensitive to light but do not form an image. 

 Reproduction is by fission or by the sexual 

 method. An animal may divide transversely; 

 each part then becomes reorganized into a 

 complete planarian. Each individual pos- 

 sesses both male and female sexual organs 

 (Fig. 68), that is, it is hermaphroditic, but 

 self-fertilization is not known to occur, and 

 cross-fertilization is certainly the rule. The 

 development is direct, without a larval stage. 

 Some fresh-water planarians show remark- 

 able powers of regeneration (Fig. 72). If 

 such an individual is cut in two, the anterior 

 end will regenerate a new tail, while the 

 posterior part will develop a new head. A 

 section from the middle of the body will 

 regenerate both a head at the anterior end 

 and a new tail at the posterior end. No dif- 

 ficulty is experienced in grafting pieces from 

 one animal to another. 



Axial gradients 



Planarians are animals that illustrate ad- 

 mirably the theory of axial gradients. The 



