PLATYHELMINTHES 183 



out" the food, and all those within a wide area collect in the pot for 

 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 of the 

 internal organs, which disappear in a regular order, the animal pre- 

 sumably using these as food. 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 dis- 

 appear, 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 re- 

 duced 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. 159, 160). 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 



