Chap. 25 FLATWORMS VANGUARD OF THE HIGHER ANIMALS 507 



duce a pharynx (Fig. 25.7). Tails thus engrafted are simply absorbed. A small 

 cross section of a planarian taken close to the head will produce a head on each 

 cut surface. The dominance of the head over the rest of the body is limited for 

 parts that are at some distance. In the natural asexual reproduction of a 

 planarian, the rear end gets beyond the control of the head and constricts 

 off as a separate; animal. Similar constriction and division can be brought on 

 by cutting off the head. All such behavior indicates that there is a gradation 

 of physiological activity from stronger to weaker and of control from front 

 to rear of the body, an anterior to posterior gradation of metabolism. 



Other Turbellarians 



Acoela. The most primitive turbellarians are the Acoela (without a cavity) 

 that have a mouth, but no definite digestive cavity. They swallow their food 

 directly into the loose mesenchyme where ameboid cells gather about it and 

 engulf the particles. Thus, digestion is intracellular like that of the amebas. 

 All the Acoela are marine, usually only one-tenth of an inch long and generally 

 little known. 



Rhabdocoela. The Rhabdocoela, named from the rod-shaped gut, are com- 

 mon throughout the world in fresh waters and along sandy and muddy sea- 

 shores (Fig. 25.8), a few in hot springs. Most of them are less than half an 



Fig. 25.8. A rhabdocoel, Stenostomum: various species of this genus are among 

 the commonest of invertebrates, cosmopolitan in standing waters but little known 

 because of their minute size. A chain of connected individuals is formed by in- 

 complete divisions of the body. (Courtesy, Morgan: Life of Ponds & Streams. 

 New York, G. P. Putnam's Sons, 1930.) 



inch long, faintly colored and little noticed. The digestive cavity is straight 

 and unbranched. The rhabdites, rod-shaped bodies of unknown function, are 

 very abundant in them. 



Tricladida. The Tricladida include land and marine planarians as well as 

 fresh-water ones. All triclads have a three parted digestive cavity. Many land 

 species live in the humid tropics, some of them marked with brilliant colors 

 and several inches long (Fig. 25.9). They are limited to localities where there 

 is a heavy rainfall, and much of the time lie under logs and leaves surrounded 

 by mucus. They travel on their own slime tracks and in tropical rain forests 

 they swing from the branches on slime threads as caterpillars swing on silken 



ones. 



Polycladida. The Polycladida have a digestive tract that is branched many 



