ACTIVITIES OF COLONIAL ANIMALS 497 
to illumination pass through it, for when one half-rachis is excited 
to glow, the other half-rachis does not follow by producing a 
flash. Recovery from this condition occurs in such preparations 
after they have been for half an hour or so in pure sea-water. 
The rate at which the luminous waves traverse the rachis of 
Renilla is a relatively slow one. It, therefore, seemed possible 
to measure it and attempts were made to carry this out by a 
photographic method, but the light that emanates from a single 
phosphorescent wave in Renilla is so faint that the most rapid 
photographic plates obtainable, even when sensitized for blue- 
green, were not fogged by it. In this test the plates were exposed 
to the light without the use of a lens and under water next the 
source of illumination. Photographic methods were, therefore, 
abandoned and an attempt was made to determine the rate by 
the use of a stop-watch and a long strip of phosphorescent tissue. 
Strips of this kind were cut from the edges of large rachides; 
they measured 5 to 8 mm. in width and about 10 cm. in length. 
When first cut they were much contracted, but in an hour or so 
they relaxed and could be pinned out each in a small wax- 
bottomed dish of sea-water. After night had come on these 
strips could be stimulated by touching one end gently with a 
.metal rod, whereupon a single wave of light would start at that 
end and pass rapidly over the length of the strip to the opposite 
end. If the stimulus was somewhat irregular, several such waves 
would pass over the preparation in rapid succession, but with a 
little attention the application of the rod to the end of the strip 
could be so regulated that a single wave was invariably called 
forth. Each wave consisted of a band of light transverse to the 
long axis of the preparation and with a sharp front edge and a 
faint rear. The width of the band of light itself in the direction 
in which it moved was 4 to6 mm. This band progressed with 
great regularity from one end of the preparation to the other, 
and its rate could be taken with fair certainty by a stop-watch. 
_ The results of measurements on five such preparations are given 
in table 4, from which it will be seen that the waves travel on 
the average 9.24 cm. in 1.25 seconds or 7.39 em. per second. 
This rate is close to the determination made on the same phenom- 
