Behavior of the lower organisms / by H.S. Jennings.

  • Jennings, H. S. (Herbert Spencer), 1868-1947.
Date:
[1906, ©1931]
Catalogue details

Licence: Public Domain Mark

Credit: Behavior of the lower organisms / by H.S. Jennings. Source: Wellcome Collection.

    82/390 (page 62)
    in Fig. 46, but in the opposite direction; it is, as it were, whirled about in its own whirlpool. The resulting path is shown in Fig. 48. This circular movement, with the oral side directed toward the centre of the circle, is seen only in specimens show¬ ing the contact reaction to objects of minute size. The contact reaction modifies strongly the reactions to most other stimuli; this is a matter which will be taken up later. Thus when Paramecium comes in contact with a solid object, it may react in three different ways. First, it may react either positively or negatively, this depending partly on Fig. 48. — Circular path followed by the intensity of the stimulus, partly Paramecium in reacting to contact with a i • i * i t.* p ,1 minute particle. on the physiological condition of the Paramecium. If it reacts negatively, this reaction may take either one of two forms. If the stimulation occurs at the anterior end, the animal gives the avoiding reaction; if it occurs elsewhere, the animal merely moves forward. 2. REACTIONS TO CHEMICAL STIMULI The reactions to chemical stimuli occur through the avoiding reaction described in the preceding chapter. As we have seen, the avoiding reaction is produced as a rule by a change from one chemical to another. With regard to this relation, there are certain facts of importance. In all cases a certain amount of change is necessary to produce reaction; that is, the chemical must be present in a certain concentra¬ tion before reaction is produced. The sensitiveness of different indi¬ viduals varies greatly, and even that of given individuals changes much with changes in the conditions. It is therefore not possible to establish for any given chemical the weakest concentration that causes the avoid¬ ing reaction. But the animals when in ordinary water are very sensitive to the common inorganic chemicals, reacting to very weak solu¬ tions. Thus the weakest solutions causing reaction have been found to be for various chemicals about as follows: — Sodium chloride, ^ to ^5- per cent (-^ to T ^ normal); potassium bromate, about per cent; sodium carbonate, about to 3^- per cent; copper sulphate, about per cent; potassium hydroxide, about