294 - Multicellular Animals, Especially Man 



EXCRETORY PORE 

 EXCRETORY VESSEL 

 GASTROVASCULAR CAVITY 

 PHARYNX 

 MOUTH 



Fig. 16-5. Planaria, a common flatworm. Diagram of a specimen with digestive 

 system injected with India ink. Note that the ink flows into all branches of the 

 gastrovascular cavity. 



side of the head there is a small blunt-pointed 

 lateral projection, the auricle, which is 

 thought to be a receptor of tactile stimuli. 



The mouth of Planaria is found at the tip 

 of a hoselike muscular organ, the pharynx 

 (Fig. 16-7). The pharynx protrudes out from 

 the ventral surface only when the planarian 

 is feeding; and at all other times it is with- 

 drawn into the pharyngeal pouch (Fig. 16-7). 



Planaria feeds upon minute organisms and 



particles of organic matter on the bottom of 

 the pond. The food passes through the mouth 

 and pharynx into the branches of the gastro- 

 vascular cavity. The full extent of this 

 branching system can be demonstrated by 

 injecting an ink suspension through the 

 mouth and pharynx by means of a hypo- 

 dermic syringe. Then it can be seen that the 

 many branches of the digestive cavity pene- 

 trate to all parts of the both 1 (Fig. 16-5). 



Fig. 16-6. This free-living flatworm {Dugesia dorotocepha/a) is closely related to Planaria. 

 It lives in fresh-water ponds and brooks, where it glides through the crevices between the 

 stones, using its cilia, which cover the underside of the body. At the head end (right) 

 note the light-sensitive "eye spots" and the touch-sensitive "auricles." Also note that the 

 protrusible pharynx can be seen (vaguely) enclosed in a pocket near the center of the 

 body. (Courtesy of J. A. Miller, Emory University.) 



The branching of the gastrovascular sys- 

 tem facilitates the distribution of the ab- 

 sorbed foods to the outlying cells of the ani- 

 mal. The body of the flatworm, compared to 

 Hydra, is considerably thicker because a 

 layer of mesoderm develops between the 

 ectoderm and endoderm (Fig. 16-8). In 

 Planaria, the mesoderm consists mainly of 

 loosely arranged parenchyma tissue, which 

 surrounds the various structures of the mus- 

 cular, nervous, reproductive, and excretory 



PLANARIA, WITH PHARYNX RETRACTED 

 IN THE PHARYNGEAL POUCH 



WITH PHARYNX EXTENDED 



Fig. 16-7. Planaria. 



