OPALIN^ID CILIATE INFUSORIANS — METCALF 561 



whose purpose is to place each species in its clearly distinct pigeon- 

 hole, but it is an instructive group. We have discussed the effect 

 of the internal habitat in removing much of the struggle for existence 

 and allowing all types that do appear to persist. This is very hkely 

 one of the chief keys to the conditions of speciation found in the family. 



Mention might be made of the rotating, spiral progress of a swun- 

 ming opalinid of any species, similar to the manner of swimming of 

 eucihates. This pattern of swimming path in the Opalinidae seems 

 to be influenced by several structural features: (l)The bend in the 

 anterior portion of the body, (2) the insertion of the cilia in spiral 

 lines, (3) the greater number and length of the ciha in front than 

 behind. These all cooperate to produce rotation, progTess, and the 

 swinging of the anterior end of the body through a wider arc than 

 that described by the posterior end. Note that the spiral position 

 of the oral groove and its emphasized ciha in the eucihates cooperate 

 in them to emphasize the spiral, rotating progress, but the mouthless, 

 grooveless opalinids have the same pattern of motion. It is a funda- 

 mental feature, and an ancestral feature, seen in Flagellata, Proto- 

 ciliata, and Euciliata. 



In many eucihates with an anterior bend in the body the oral 

 groove and mouth is in the region of the bend. One gets the im- 

 pression that the mouth of the ancestors of the Opalinidae may hkely 

 have been near the bend. Detailed study of the arrangement of the 

 "neuromuscular" fibrillae would be worth while, to see if there is any 

 asymmetry or any "center" in this region to make this interpretation 

 probable. 



In a previous pubhcation (Metcalf, 1923a) I gave a table of the 

 hosts of the known Opahnidae and of the geographic occurrence of the 

 opahnids. The following table presents the new data since that 

 pubhcation. 



166877—40- 



