208 



THE INVERTEBRATA 



the feast. When the animal is too large to be ingested whole, the 

 pharynx is attached to the prey and worked backwards and forwards 

 with a pumping motion, while at the same time a disintegrating 

 digestive fluid is poured out from the walls of the pharynx. Particles 

 of food are thus pumped up into the gut cavity and digested in the 

 same way as the living prey. In the Trematoda, also, the cells lining 

 the gut have a certain limited power of amoeboid movement at their 

 exposed edges, and intracellular digestion is apparently the usual 

 method. 



The Turbellaria are able to go without food for long periods, but 

 during starvation they grow smaller and smaller. Stoppenbrink 

 starved Planaria alpina, keeping them entirely without food, while as 

 a control he kept a similar collection supplied with food. His results 

 are given in the table below. The measurements are in millimetres. 



This reduction in size is accompanied by the absorption and 

 digestion of the internal organs, which disappear in a regular order, 

 the animal using these as food in the manner already described. The 

 first things to go are the eggs which are ready for laying, then follow 

 the yolk glands and the remainder of the generative apparatus. 

 Finally the ovaries and the testes disappear, so that the animal is 

 reduced to sexual immaturity. Next the parenchyma, the gut and the 

 muscles of the body wall are reduced and consumed. The nervous 

 system alone holds out and is not reduced so that starved planarians 

 differ in shape from the normal forms in having a disproportionately 

 large head end, the bulk of which is the unreduced cerebral ganglion. 

 On feeding these starved forms will regenerate all the lost organs and 

 return to the normal size, like Alice when she ate the right half of the 

 mushroom. 



It is in the generative organs that the Platyhelminthes show the 

 greatest complexity of organization (Figs. 165, 166). With rare excep- 

 tions the Platyhelminthes are hermaphrodite. The generative pore is 

 variably placed but it is usually to be found in the middle line of 

 the ventral surface not nearer to the anterior or posterior end than 



