FIELD BOOK OF PONDS AND STREAMS 



food canal go in all directions making every region capable 

 of digesting food and growing. If a planarian is cut crosswise 

 (Fig. 95) the head will grow or regenerate a new body and the 

 body a new head as they do in natural transverse division. 

 If one is cut lengthwise through the center of the body the 

 right side will regenerate a new left and vice versa. If no 

 food is given to these regenerating animals the growing side 

 will live on the older part and the whole animal will be 

 dwarfed. 



Identification. — On the outside of their bodies turbellarians 

 have few external features by which the different species 

 can be separated. Their classification is based very largely 

 upon the sex organs which are undeveloped in all but the fully 

 grown animals and difificult to see at any time. The main 

 groups, however, are divided according to the shape of the 

 intestine, in this case m.eaning all of the food tract except the 

 pharynx, and often clearly visible through the transparent 

 body wall (Fig. 93, 2), Fresh water turbellarians have a 

 single, blind intestine as in the Order Rhabdoccelida or a three- 

 branched one as in the Triclada. 



I. Turbellarians with a single, blind intestine. Order 

 Rhahdocoslida. 



Fig. 96. — Stenostomum leiicops, showing the chain 

 of individuals produced by automatic division of 

 the body. 



Stenostomum leucops. — This is a very common species 

 (Fig. 96) which may be here taken as a type of the rhab- 

 docoeles. These minute flatworms are common on plants in 

 quiet ponds, but are never more than a quarter of an inch 

 long, so small that they are seldom seen and even less often 

 recognized. They reproduce by budding, a kind of transverse 

 division, and their best recognition marks are the chains of 2 

 to 4, rarely 9, animals attached to one another, tandemwise. 



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