BEHAVIOR UNDER NATURAL CONDITIONS 181 



species living under similar conditions. Some of the free-swimming 

 species are very supple, changing form continually. Such is the case, 

 for example, with Lacrymaria olor, which stretches its long neck in 

 every direction, shortens it until it has almost disappeared, reextends it, 

 and seems to explore thoroughly the surrounding region. Such an 

 organism has, of course, much better opportunity for effective behavior 

 by the method of trial than has such a rigid form as Paramecium. 



Similar differences are found among the creeping infusoria, and 

 among the fixed species. Some fixed infusoria contract frequently, 

 while others contract only rarely. In some cases the contraction occurs 

 at regular intervals, even when there is no indication of an external 

 stimulus. This is the case with Vorticella. There is no evidence that 

 in infusoria periods of rest, comparable with the sleep of higher animals, 

 are alternated with periods of activity. Hodge and Aikins (1895) 

 kept a single Vorticella continuously under observation for twenty-one 

 hours, besides intermittent study for a number of clays. They found 

 that there was no period of inactivity. During five days the cilia were 

 in continuous motion, food was continuously taken, and contractions 

 were repeated at brief intervals. 



A number of fixed infusoria live, like Stentor rceselii, in tubes, some 

 gelatinous, some membranous in character. As a rule these tubes are 

 formed in a very simple manner. The material of which they are com- 

 posed is secreted by the outer surface of the animal. In the repeated 

 contractions and extensions of the body this material is worked off, in 

 the form of a sheath. The tube may become thicker by the secretion 

 of more material on the surface of the animal. It often grows in length, 

 either as the animal becomes longer or as it migrates farther out toward 

 the open end of the tube. In the secreted material, which is often trans- 

 parent, all sorts of foreign substances may become embedded, in the 

 following way: They are carried as particles to the oral disk by the cilia. 

 Thence they pass backward over the surface of the body, till they reach 

 the gelatinous substance of the tube, where they become embedded. 

 Thus in most cases the formation of the tube seems a direct consequence 

 of the secretion of the mucus-like substance over the body of the animal, 

 taken in connection with the usual movements. The intervention of any 

 special type of behavior directed toward the end of forming the tube 

 seems unnecessary. But in some cases, as we have seen in our account 

 of Stentor, the tube is formed at the beginning by a definite set of move- 

 ments, of a character especially fitted to produce such a structure. For 

 details as to different kinds of tubes, and their structure and method 

 of formation, reference may be made to Butschli's great work on the 

 infusoria (1889). 



