HOLOTHUEIA. 



203 



over the whole surface of the animal, and are protruded through count- 

 less minute orifices that perforate the integument. In other cases, as 

 in H. frondosa, they are arranged in five series, resembling the ambu- 

 lacra of an Echinus ; and in some instances they are only found upon 

 the middle of the ventral surface of the body, that forms a flattened 

 disk upon which the animal creeps, somewhat in the manner of a snail. 



Fig. 100. 



Holothuria. 



The ambulacral feet themselves, represented on an enlarged scale at c, 

 precisely resemble in aU the details of their structure those of the 

 Asterias, and their protrusion and retraction are effected in the same 

 manner; but, in addition to these organs, we find in some genera move- 

 able hooks or spines (fig. 100, d\ which are likewise retractile, and 

 most probably assist in locomotion. 



