66 



IRRITABILITY 



only the extreme end lashes with regularity through the water 

 (Figure 8, A). There is majestic grace in this perfect uniformity 

 of motion. The picture suddenly alters the moment the Peranema 

 is influenced by the slightest jar. The whole flagellum at once 

 executes a few violent movements (Figure 8, B), the body draws 

 together, soon stretches itself again and swims immediately after, 

 in another direction, with the same majestic calm as before. 



Fig. 8. 



Peranema, A — Swimming in non-stimulated condition. 

 B — Mechanically stimulated at the end of the flagellum. 



Another instance. A number of fertilized eggs of the sea 

 urchin are placed in a watch glass in sea water. The temperature 

 of the water should correspond with the mean temperature in 

 which the animals live in the sea, averaging about 15° C. The 

 eggs begin to form grooves and to develop slowly by progressive 

 division. In another glass we observe a second sample of fer- 

 tilized eggs of the same kind and under the same conditions, but 

 in this case we increase the temperature to 25° C. The increased 

 temperature brings about a decided increase of segmentation and 

 the same stage of development is reached in less than half the 

 time. The increased temperature, therefore, increases the devel- 

 opment. Further we take a third sample of the same urchin 

 eggs in a watch glass with sea water of 15° C. and add a little 



