286 GENERAL BIOLOGY 



The digestive system consists of the mouth, proboscis or pharynx 

 (which lies in a muscular sheath), and three chief interior intestinal 

 branches, one running forward to the head end of the body and two 

 leading tailward. Many small side pouches or diverticula protrude. In 

 fact, every part of the body has such a pouch. This means that all 

 parts of the body can take nourishment immediately from the digestive 

 tract so that planaria needs no circulatory system. All non-digested 

 food must be egested through the mouth, as there is no anal opening. 



In some forms a definite green substance appears which is due to 

 the zoQchlorellae or symbiotic one-celled plants which live in the middle 

 .germ-layer. 



Fopd is digested both intercellularly and intracellularly, which 

 means that a part of the food is digested in the intestine proper by secre- 

 tions which are poured out 

 from cells in the intestinal 

 walls; and, that food may also 

 be digested by pseudopodia 

 extending from cells in the in- 

 testinal walls, which pseudo- 

 podia take in the undigested 

 food to the cell which then 

 B. digests it. 



Fig. 175. A. Planaria polychroa X about 4. 



1, Eye. 2. Ciliated slit at side of head. 3. Mouth External Annearanre 



of proboscis. 4. Outline of the pharynx sheath into ^ Xierr ince - 



tTv h e Ch por h e e pharynx * withdrawn - 5 ' Reproduc - Planaria is bilaterally 



B. Dendrocoeium 9 ra ffi . (Woodworth). symmetrical and dorso-ven- 



trally flattened. The head-end is blunt and the tail-end tapers. It is 

 usually less than half an inch in length. The common American species 

 is known as Planaria maculata. It has a definite pair of eye-spots. 



Turbellaria are metazoans and triploblastic. The mesoderm con- 

 sists mostly of muscles and loose parenchyma cells. The coelom is rep- 

 resented by the genital sacs. 



Turbellaria are classified according to the type and number of 

 .branches found in the digestive tract. 



In some of the turbellaria, though not in Planaria, there are special 

 'ectodermal cells which secrete mucus, or produce rod-like bodies called 

 rhabdites. 

 The Excretory System. 



The excretory system (Fig. 176) consists of two irregular longi- 

 tudinal much-coiled tubes, one on each side of the body. Near the an- 

 terior end these two tubes are connected by a transverse vessel. The 

 longitudinal vessels open to the exterior by two small pores on the 

 dorsal surface of the animal. 



Many fine tubules branch off from these main tubes and ramify 

 through all parts of the body, terminating in large flame-cells (Fig. 177). 



