Phyla Platyhelminthes and Nemertina 407 



Planaria is well supplied with sense organs which are concentrated 

 along the sides of the body and in the head region. Most conspicuous 

 of these are the light-sensitive eyes. Each is formed of a series 

 of light-sensitive cells lining a cuplike pigmented structure. The light- 

 sensitive cells are bipolar and have expanded ends projecting into the 

 pigmented cup. The "ears" or auricles are well supplied with sensory 

 cells and are probably sensitive to both chemical and tactile stimuli. 



Like most flatworms, the planarians are hermaphroditic. The 

 sperm are produced by several hundred minute, spherical testes located 

 along both sides of the worm. Each testis has a small vas efferens 

 which connects to the main vas dejerens. There is one vas deferens 

 on each side of the body and these two enter the seminal receptacle 

 which is located near the center of the animal. This connects to the 

 muscular penis which in turn opens into the genital atrium just beneath 

 the genital pore. 



The female system consists of a pair of ovaries near the anterior 

 end of the animal. These connect to two oviducts which parallel the 

 nerve cord along each side. The oviducts join in the median vagina 

 which also opens into the genital atrium. A median seminal receptacle 

 or copulatory sac opens into the vagina. Along the oviducts are many 

 yolk or vitelline glands which supply the developing eggs with yolk cells. 



The Behavior of Planaria. — On the whole, in its reactions to 

 factors in its environment, planaria shows a much higher degree of 

 coordination than does hydra. Weak stimuli of varying types result 

 in approaching reactions, while strong ones result in avoiding reactions. 

 They avoid strong illumination and are most active at night. 



The ventral cilia aid in the gliding movements of these animals. 

 They move along with the head end slightly raised, and the cilia beat with 

 a backward stroke against the slime trail left by the glandular secretions. 

 Planarians also crawl along a substrate by attaching the anterior end with 

 slime and then bringing the posterior end forward by muscular con- 

 tractions. In an aquarium they are most often seen moving over objects, 

 along the glass walls, and on the undersurface of the water film. 



Planaria is able to detect food within its vicinity. If small pieces 

 of meat are placed in an aquarium, the planarians will soon gather and 

 start feeding. The pharynx actually sucks the small pieces off and 

 passes them into the intestine. Within the intestine, digestion is intra- 

 cellular. There are enzymes present only for the digestion of animal 



