482 THE AMERICAN NATURALIST. [Vor. XXXV. 
plates; five serve as points of attachment for the radial muscles 
and are called radial pieces, while the five which alternate with 
them are called zwzerradial. The size and shape of these 
pieces differ greatly, the most important difference being 
whether they have posterior prolongations (Fig. 7) or not 
(Fig. 8). The tentacles around the mouth vary greatly in 
number and appearance. There are usually ten or twenty, but 
species are known which have normally eight, eleven, twelve, 
fifteen, eighteen, and twenty-five, and in individual cases the 
number runs up to thirty. In some species the number is very 
constant, and individual diversity is quite rare ; while in other 
species the amount of diversity is extraordinary, ranging in 
some species of Holothuria which have normally twenty tenta- 
cles, from twelve to twenty-nine. In appearance the tentacles 
may be simple, finger-like, and unbranched, but this condition 
is very uncommon. Usually each tentacle has several to many 
branches ; when these all arise in a tuft from near or at the 
tip and are more or less subdivided, the tentacle is called 
peltate (Fig. 10); when the branches occur regularly along the 
sides in two opposite series and without subdivisions, the ten- 
tacle is pinnate, or, if the branches are very few and all arise 
from the tip, digitate (Fig. 11); when the branches are sub- 
divided and irregularly arranged the tentacle is said to be 
dendroid (Fig. 9). In some species these tentacles are used 
for locomotion, but they are more commonly used as feelers 
and to carry food to the mouth. Locomotion is generally 
accomplished by means of ambulacral- or tube-feet, as in the 
other echinoderms. These feet, when present, may be 
arranged in regular rows along the five radii (ambulacra), or 
scattered more or less irregularly over the whole body surface. 
Frequently those on the upper side of the animal are very 
different in size, appearance, and structure from those on the 
under or ventral side, and no longer serve for locomotion, the 
terminal sucking disk being generally absent. They are then 
called papille, the true-feet being pedicels. Rarely, all the 
tube-feet are papilla and no pedicels normally occur. The 
papilla are often situated on warts or tubercles, and these are 
sometimes of considerable size. The pedicels are frequently 
