ON THE LOCOMOTOR SYSTEM OF ECH1NODERMATA. 
843 
the pedicels are confined to the ventral surface of a flat-shaped organism, in the 
Echinus they are protruded from all sides of a globe. That they are habitually used 
as feelers is evident from watching tlieir movements. For instance, when an Echinus 
is crawling along a flat horizontal surface, the rows of pedicels facing the direction of 
advance are more strongly protruded than those of the other rows ; although none of 
the pedicels from some distance below the equator are in use for walking, in the rows 
mentioned they are extended to their fullest length, in order to feel for any object 
which the animal may possibly be approaching. On the other rows only a single 
pedicel here and there is thus fully extended ; such, however, no doubt also act as 
feelers, to warn the Echinus of the approach of any object from behind or from the 
sides. When a perpendicular surface is reached, the animal may either ascend it or 
crawl along for an indefinite distance, feeling it all the way with its pedicels. It may 
here be added that when an Echinus starts walking, it generally keeps pretty per¬ 
sistently in one direction of advance. If it be partly rotated by the hand, or other 
external means, it does not continue in the same direction, but continues its own 
movements as before; so that, for instance, if it has been turned half round, it will 
proceed in a direction opposite to that in which it had been proceeding before its rota¬ 
tion. When fresh specimens are at rest, a certain small percentage of feet are used as 
anchors. The others are strongly protruded on all sides as feelers ; but in specimens 
not quite fresh, nearly all the feet not in use as anchors are retracted, with only one 
here and there protruded as a feeler. 
When an Echinus is inverted upon its ab-oral pole, its shape renders execution of 
the righting manoeuvre a much more difficult matter than is the case in the analogous 
position of a Star-fish ; for while a Star-fish is provided with flat, flexible, and muscular 
rays, composing a small and light mass in relation to the motive power, an Echinus 
is a rigid, non-muscular, and globular mass, whose only motive power available for 
conducting the evolution is that which is supplied by relatively feeble pedicels. It is 
therefore scarcely surprising that unless the specimens chosen for these observations 
are perfectly fresh, they are unable to right themselves at all; they remain per¬ 
manently inverted till they die. But if the specimens are fresh, they sooner or later 
invariably succeed in righting themselves, and their method of doiug so is always the 
same. Two, or perhaps three, adjacent rows of pedicels are selected out of the five, 
as the rows which are to accomplish the task (Plate 82, fig. 23). As many feet upon 
the rows as can reach the floor of the tank are protruded downwards and fastened 
firmly upon the floor; their combined action serves to tilt the globe slightly over in 
this direction—the anchoring feet on the other, or opposite, rows meanwhile releasing 
their hold of the tank floor to admit of this tilting (Plate 82, fig. 24). The effect of 
the tilting is to allow the next feet in the active ambulacral rows to touch the floor of 
the tank, and when they have established their hold, they assist in increasing the tilt; 
then the next feet in the series lay hold, and so on, till the globe slowly but steadily 
rises upon its equator (Plate 83, fig. 25). The difficulty of raising such a heavy mass 
5 q 2 
