205] TURBELLARIA FROM THE MISSISSIPPI BASIN— HIGLEY 11 



rigidity, many details are clearer in the live tissue than in permanently mounted 

 material, where a large amount of cellular contraction is unavoidable. Many 

 details of structure, however, especially those of the nervous system, are clear 

 only from a study of prepared material. Serial sections in the several planes 

 have been used to verify and correlate findings in the various body systems. 

 Some Uttle difficulty was experienced in an attempt to make a detailed 

 study of the tissues. Their extreme contractility and sensitiveness make it 

 almost impossible to kill and fix material in a condition any^^hcre near the 

 normal. The dehcacy of the structure as a whole as well as of every part, 

 very much increases the chance of injury and even when great care is usedj the 

 tissues are generally somewhat torn and distorted. The integument is so 

 easily ruptured that often only the weakest anesthetizing fluids can be used, 

 and then complete quiet is not obtained. Then also the species which repro- 

 duce by budding break very easily at the fine of division and are likely to be 

 mutilated. As a consequence of this, especial gentleness and care must be 

 used in the handling and control of all specimens. 



