96 PLATYHELMINTHES [CH. 



This pharynx can be withdrawn into the body or pushed a little 

 way out of the mouth. The chamber in which it lies, and which 

 gives it room to play in and out, is an ectodermal pouch called the 

 pharynx-sheath. This is small in Mesostoma, but in other Tur- 

 bellarians it may be much larger, and the pharynx is consequently 

 capable of stretching out a long distance. This is the case, for 

 instance, with the fresh-water genus Planaria (Fig. 42). 



Certain glands, called salivary glands, open into the cavity of 

 the alimentary canal at the inner end of the pharynx, and then the 

 cavity opens into the stomach, which is a sac-like structure with 

 no other opening than the mouth lying in the centre of the body. 

 The cells lining this cavity are, like the endoderm cells of Hydra, 

 amoeboid, and they take up particles of food into themselves in the 

 same way that an Amoeba does. This primitive form of digestion 

 has been lost in most of the higher animals, where the digestive cells 

 pour out solvent fluids into the cavity of the alimentary canal, and 

 the food is rendered soluble in this cavity before being absorbed. 

 In the Turbellaria, as in the Coelenterata, this secondary method of 

 digestion coexists with the amoeboid method. 



Mesostoma is carnivorous and eats small worms, minute crus- 

 tacea and insect larvae. It uses its mucus to ensnare and entangle 

 its prey. Its method of devouring them resembles the habits of 

 the Starfish. It holds them fast by means of the pharynx, using 

 this as a sucker. The so-called salivary glands secrete a strong 

 digestive ferment which rapidly dissolves the flesh of the victim, 

 reducing it partly to a fluid condition and partly to a disintegrated 

 mass of particles. There is no vascular system to distribute the 

 digested food to the different parts of the body, so that these 

 products must be passed from cell to cell through the solid body 

 until they arrive where they are needed. The undigested parts are 

 passed out through the mouth. 



The two main ducts of the excretory or "water- vascular" system 

 open near the sides of the mouth, each then passes upwards towards 

 the dorsal surface and divides into two longitudinal vessels, one 

 running towards the head, the other towards the tail. These four 

 longitudinal branches give off innumerable finer ones, which sub- 

 divide until each branchlet ends in a flame-cell. These latter are 

 very minute and require a high power of the microscope and very 

 careful focusing to see. "We may thus say that the excretory 

 system consists of a pair of very greatly branched nephridia. 



The nervous system consists of a large ganglion called the bruin, 



