160 



METAZOAN PHYLA 



hitherto studied. The mesoderm is well developed, forms the greater 

 part of the mass of the body, and is composed of a meshwork of living 

 cells inclosing small spaces and known as the parenchyma. This paren- 

 chyma is covered externally by the epidermis. In it are imbedded 

 a variety of other structures which make up more or less definitely 

 organized systems. 



One of these systems is the digestive system (Fig. 77 A). A mouth 

 opening leads into a mouth, or buccal, cavity, which contains a protrusible 

 pharynx. The wall of the pharynx forms a fold which projects forward 



Enferon 



Pharynx 



Opening oF 

 pharynx 



Mouth 



Cephalic 

 ganglion 



Excretory 

 tubes 



Excretory 

 pore 



Nerve 

 cord 



Peripheral 

 nerve 



ABC 



Fig. 77. — Systems of a planarian. Somewhat diagrammatic. A, digestive system. 

 B, excretory system. C, nervous system. {From Parker and Haswell, "Text-book of 

 Zoology," after Jijima and Hatschek, by the courtesy of The Macmillan Company.) 



into that cavity for nearly its full length and at its outer end is entirely 

 free. When protruded beyond the mouth opening, the pharynx forms 

 a proboscis. The pharyngeal cavity leads into another cavity called 

 an enter on^ though sometimes referred to as an intestine. This consists 

 of three main trunks, one anterior and two posterior and lateral, each 

 of which has a large number of blind extensions, the whole representing 

 a very complicated gastr avascular cavity. 



