131 



Cf 



CLASS I—TURBELLARIA 



These are almost all free-living, and retain the gut, though 

 this may be reduced and solid. The outer layer of the body 

 consists of cells covered with cilia, by which the animals move 

 on surfaces, although many of them also use muscles for this 

 purpose and for swimming. In or just below the ectoderm are 

 usually scattered cells containing peculiar rod-shaped bodies or 

 rhabdites, of unknown function. There are sense cells of various 

 sorts, mostly in the anterior part of the body, and 

 usually eyes, which have the retina well-marked, 

 but no lens. The nervous system (Fig. 94) is 

 interesting and shows the beginning of a brain. 

 There is a pair of ganglia in the head, united by 

 a commissure and giving off longitudinal nerves 

 to the body. These supply a nerve net under the 

 epidermis and another deeper in the body. The 

 function of this brain is merely to relay and distri- 

 bute the impulses from the important sense organs 

 on the head. Unlike Hydra, where no part of the 

 body permanently dominates the rest, these 

 creatures, moving as they do always with the same 

 end forwards, have that end organised for 

 perception and the constant stimulation of the 

 rest of the body. This permanent organisation 

 of a dominant region of the body unifies the 

 reaction of the body as a whole to changes in its 

 surroundings. But in the Turbellaria the brain does 

 not co-ordinate the activities of different regions of 

 the body. It sets them in action : that each plays 

 its proper part is due to local organisation. 



The Turbellaria are mostly carrion feeders, and protrude a 

 long muscular pharynx, into which the food is sucked (Fig. 95). 

 Two interesting features of them are their ability to regenerate 

 when cut into pieces, and the fact that when starved they degrow. 

 Not only do they decrease in size, but the internal organs are 

 reduced and disappear in order, first the eggs and reproductive 

 organs, and then the gut and muscles, the nervous system alone 

 being unreduced. When given food these starved animals 

 regenerate their organs and return to their normal size. 



The commonest British species of Turbellaria all belong to the 



Fig. 94. — A 

 diagram of the 

 nervous system 

 of a turbellarian, 



e.g., brain ; e., eye; 

 /.«., longitudinal 

 nerve cord ; o., 

 opening through 

 which the pharynx 

 is protruded ; ph., 

 pharynx ; ph.s., 

 pharynx-sheath. 



