Ii6 BEHAVIOR OF THE LOWER ORGANISMS 



lation, the animal responds by trying many new directions, till one is 

 found which does not lead to stimulation. The reaction is less flexible 

 in the ciliates which creep along surfaces than in the free swimming 

 ones. In the former, owing to the lack of revolution on the long axis, 

 all the directions tried lie in a single plane. But under many powerful 

 stimuU even these species usually leave the surface on which they are 

 moving; they then react in the freer way characteristic of unattached 

 organisms, trying directions lying in many different planes. 



There exists also a large number of cihates which become more or 

 less permanently attached by the part of the body opposite the mouth. 

 This attached portion is usually drawn out to form a slender stalk or 

 foot. Examples of such infusoria are Stentor (Fig. 31, b) and Vor- 

 ticella (Fig. 31, c). Some of these species are found attached under 

 all usual conditions ; such are Vorticella and Carchesium. Others are 

 frequently found swimming freely; this is the case, for example, with 

 Stentor caruleus. Some infusoria become fixed in only a temporary 

 way, by a mucous secretion. Such are Spirostomum and Urocentrum, 

 which are often found suspended from solid objects by a thread of 

 mucus (Fig. 82). Even the species which are most firmly fixed may 

 under powerful stimuli detach themselves and swim away. The heads 

 of Vorticella and Carchesium thus at times detach themselves from their 

 stalks and swim about like Paramecium. At such times they may also 

 creep over surfaces, just as do the Hypotricha. The behavior when free 

 is essentially similar in its main features to that of Paramecium or 

 Oxytricha. 



In the attached condition the mouth and peristome are usually 

 above, surrounded by a wreath of large cilia. These cilia are in con- 

 tinual movement, in such a way as to bring a current of water from above 

 to the mouth. Some fixed infusoria contract at intervals with marked 

 regularity, even when there is no external stimulation. Such is the case 

 in Vorticella. The reactions to stimuli are much modified as compared 

 with those of the free swimming species. The avoiding reaction be- 

 comes broken up into a number of factors, any one of which may take 

 place more or less independently of the others. Thus, Stentor rasselii 

 may respond to stimulation either by a reversal of the cilia, driving away 

 the water currents, by bending over toward the right aboral side, or by 

 withdrawing into its tube. Each of these reactions corresponds to a 

 certain definite feature in the avoiding reaction of free infusoria. Owing 

 to the disintegration of the avoiding reaction into independent parts, the 

 behavior of these fixed infusoria become more varied and more highly 

 developed than that of the unattached species. We shall have occasion 

 to treat of this in detail later, in our account of the modifiability of 

 reactions in Protozoa. 



