208 ^Wf ANIMAL KINGDOM 



but can detect roughly the amount of light and the general direction 

 from which it comes. 



In a water current planarians usually face or crawl upstream. Cur- 

 rent direction is recognized by tactile fibers, scattered along the sides 

 of the animal, which are bent by the force of the water. 



All of this sensory information, especially that from the head re- 

 gion, is relayed by nerve fibers to the brain, a bilobed white structure 

 between the eyes (Fig. 11.1). Nerves branch out in all directions from the 

 brain; the primary pair are the ventro-lateral nerve cords (Fig. 11.4). 

 ff the brain is removed coordination is seriously impaired, and almost 

 all the relation of sensory information to locomotion is lost. 



The brain controls both ciliary and muscular action. If a planarian 

 is bumped, ciliary locomotion ceases at once and the body contracts, 

 withdrawing the end that was touched. If touched repeatedly on the 

 tail, Dugesia will hasten forward by a series of wormlike body con- 

 tractions. If touched repeatedly on the head, it will back up, turn to 

 one side, and go forward again. 



Muscles for these movements lie beneath the epidermis (Fig. 11.4). 

 Outer circular fibers can constrict and lengthen the body, while deeper 

 longitudinal fibers can shorten it. Other fibers are oblique and still 

 others are vertical. The latter can flatten the body. Coordination among 

 these fibers is such that the planarian can accomplish a number of ma- 

 neuvers, turning, folding or stretching in all directions. When the 

 organism is gliding smoothly along the bottom, successive waves of 

 contraction of the longitudinal fibers may pass from the posterior end 

 to the front, considerably increasing the rate of locomotion. 



75. Dugesia: Water Balance and Excretion 



The remainder of the flatworm body, the space between muscles and 

 intestine, is filled with loosely organized mesodermal cells, the mesen- 

 chyme. Some of these cells are pigmented, giving the worm its char- 

 acteristic brown or gray color. The mesenchyme forms a loose mesh 

 containing a considerable amount of intercellular fluid that flows back 

 and forth as the worm changes shape. The movement of this fluid prob- 

 ably aids in the distribution of nutrients from the intestine to other 

 parts of the body. 



Excess water from the body cells diffuses into the intercellular 

 fluid and is picked up by excretory cells. These are the protonephridia 

 or flame cells (Fig. 11.6, A) scattered throughout the body. Each flame 

 cell surrounds a blind tubule into which the water is excreted. A tuft of 

 cilia in the blind end beats vigorously, propelling the fluid down the 

 lumen. These tubules from the protonephridia empty into larger tubules 

 that form an anastomosing system along each side of the body (Fig. 

 11.6, B). These open to the surface through numerous small pores. 



The number of flame cells in the body is adjusted to the salinity 

 of the environment. Planarians grown in slightly salty water develop 

 few flame cells, but quickly increase the number if the amount of salt 

 is later reduced. 



