Flatworms and Threadworms 



287 



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 need 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, due to the 

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

 germ-layer. 



Food is digested both intercellularly and intracellularly, which 

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



secretions poured out from cells 

 in the intestinal walls ; and, 

 that food may also be digested 

 by pseudopodia extending from 

 cells in the intestinal walls. In 

 the latter case the pseudo- 

 podia take in the undigested 

 food to the cell which then 

 digests it. 

 External Appearance. 



Planaria is bilaterally 

 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 turbellaria, though not in planaria, there are special ecto- 

 dermal 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 

 anterior 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). 



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



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

 of proboscis. 4. Outline of the pharynx sheath into 

 which the pharynx can be withdrawn. 5. Reproduc- 

 tive pore. 



B. Dendrocoelum graffi. (Woodworth.) 



