ACTIVITIES OF COLONIAL ANIMALS 345 
upper surface as the colony rests on the sand would be ventral, 
the lower dorsal, a perversion that recent writers have been dis- 
inclined to follow. ‘To avoid confusion in this respect, I shall, 
therefore, not use the terms dorsal and ventral, but I shall call 
the surface of the Renilla rachis that is uppermost in the resting 
position and that carries the zodids superior and the opposite 
one inferior. 
Renilla amethystina is commonly found in sand banks between 
high and low tide. When such a bank that has been covered 
for some time with a foot or so of water is closely examined, 
many of the heart-shaped colonies of this species will be dis- 
covered fully expanded and spread out upon the surface of the 
sand (fig. 1). In Renilla amethystina the maximum diameter of 
the colony may reach 8.5 cm. and the largest zodids may rise 
from the surface of the rachis to the height of 5 or 6 mm. Not- 
withstanding the fact that Renilla is so bright a purple as to 
justify the popular name of sea-pansy, the colony in its resting 
position is so commonly covered with a thin layer of sand as to 
make it easily overlooked. This peculiarity is enhanced by the 
fact that the zodids, which are almost always well above the level 
of the sand, are transparent or at most slightly grayish in tint. 
As the tide recedes Renilla contracts and withdraws gradually 
into the sand so that with the disappearance of the last of the 
water the animal leaves a mark on the sand not unlike that of a 
miniature horseshoe (fig. 2). These marks serve as sure indi- 
cations to the collector of the presence of Renilla. They are 
quickly obliterated by the returning tide, whereupon Renilla 
reexpands to assume the form already described. These tidal 
responses were observed as early as 1864 by Fritz Miller. 
The expansion and contraction of Renilla is accomplished, as 
might be expected, by taking in and discharging sea-water. 
Louis Agassiz (’50, p. 208) long ago observed that an expanded 
Renilla reniformis might thus temporarily have a diameter double 
that of its contracted state, and the same seems to be true of 
Renilla amethystina. A specimen of this species (fig. 3), whose 
rachis when fully expanded had a diameter of 6.5 em. was made 
to contract completely (fig. 4), after which its diameter was found 
