172 



Aquatic Organisms 



transverse division of the body, the division plane lying 

 close behind the mouth. When a new head has been 

 shaped on the tail-piece, and a new tail on the head- 

 piece, and two capable organisms have been formed, 

 then they separate. In some of the simple (Rhab- 

 doccele) flatworms the body divides into more than two 

 parts simultaneously and thus chains of new individuals 

 arise (fig. 79 c). 



Thread-worms or Nematodes, abound in all fresh 

 waters, where they inhabit the ooze of the bottom, or 

 thick masses of vegetation. They are minute, color- 

 less, unscgmented, smoothly- contoured cylindric worms 

 rarely more than a few millimeters long. The tail end 

 is usually sharply pointed. The mouth is terminal at 

 the front end of the body, and is surrounded by a few 

 short microscopic appendages. Within the mouth 

 cavity there are often little tooth-like appendages. 

 The alimentary canal is straight and cylindric and 

 unappendaged, and the food is semifluid organic sub- 

 stances. 



Fig. 



Diagram of a Nematode wc 



w;, mouth; k, ner\'C ring; f, alimentan' canal; ov. 

 Of, ovaries; (i, anu^. (After Jayerskiold). 



Wo can hardly collect any group of pond-dwellers 

 without also collecting nematodes. They may occupy 

 any crevice. They slip in between the wing- pads of 

 insect nymphs, and into the sheaths of plant stems. 

 When we disturb the trash in the bottom of our collect- 

 ing dish, we see them swim forth, with violent swings 

 and reversals of the pliant body. They mav easily be 

 picked up with a pipette. 



