206 ^^^ ANIMAL KINGDOM 



Pharynx rctra-cbe-d 



MoTxth cavity 



Mouth- 



'■■'mm 



PhsLrynx 



e-jctended" 



Figure 1 1 .3. Diagrammatic side view of the pharynx of Dugesia retracted (left) and 



extended through the mouth (right) 



Vertical inuscle 



"Muscle layers 



Secondary 

 intestinal branches 

 Mesenchyme 



, Venlro-laleral nerve 



Adhesive glands 



Pharynx 



Moath cavity 



Figure 1 1 .4. Cross section of Dugesia at level of the pharynx. 



Ciliated 

 epidermis 



Primary branch 

 of intestine 



planarian folds over the prey. After sliding around the prey once or 

 twice, binding it tightly in slime and glue, the planarian comes to rest 

 with the anterior half of its body on the bottom and the posterior half 

 doubled over the prey. 



The mouth of Dugesia is midventral (Fig. 11.1). The pharynx is a 

 long extensible tube which can pass through the mouth. When not in 

 use, the pharynx is withdrawn into a mouth cavity lined with ectoderm 

 (Fig. 11.3). The pharynx itself is covered with ectoderm, and its wall is 

 composed of several layers of muscle and connective tissue (Fig. 11.4). 

 When withdrawn the pharynx is short and stout, but by contraction of 

 the circular muscle fibers it can be elongated greatly (Figs. 11.2 and 

 11.3). When feeding it is extended and used as a probe to search the 

 prey for a tender spot. It then bores into the prey by strong sucking 

 movements and tears the soft parts to bits to be swallowed. 



After a meal the planarian crawls off a short distance and rests, 

 with the body rounded up and firmly attached to the bottom by glue 

 from the marginal adhesive glands. 



The pharynx opens into a branched intestine (Fig. 11.1), one 

 primary branch extending into the head, and two more extending 

 toward the tail. All have side branches so that in a cross section of the 

 worm the intestine may be cut across several times. As in the coelen- 

 terates and ctenophores, the digestive organ is a simple gastrodermis of 

 endoderm cells. In planarians these cells are very large, making the 

 intestine the bulkiest structure in the body. Although no digestive 

 enzymes have been found in the lumen of the intestine, it is obvious 



