ECHINODERMATA. PAs 
advance either in water, over small spaces, or up the vertical face of 
rocks. The ambulacral feet are very numerous, disposed in rows 
along the under surface of the rays ; thus, in A. aurantiaca there are 
two simple rows of ambulacral feet attached to each ray, and the 
vesicular part is deeply cleft into two, lobes ; while in A. rudens (Fig. 
105) there are two double rows of ambulacral feet on each ray, and 
each ambulacral foot has at its base a closed ambulacral vesicle. 
Each of these ambulacral feet consists. of two parts, an internal 
and generally bladder-like portion placed within the body, and a tubular 
portion outside, projecting from the surface through an aperture in 
the skin or shell, the tube being closed at the extremity, and 
terminating in a sucker, usually in the form of a disc slightly 
depressed in the centre; around the margin of this sucker-like 
extremity will be often found pretty rosette-like shaped calcareous 
plates, better seen in the Echinide, The feet are thus muscular 
fleshy cylinders, hollow in the centre, and very extensible ; by means 
ofthem the animal draws itself forward. The foot is extended by the 
contraction of its internal ambulacral vesicle, which forces the 
ambulacral fluid into the hollow tube, or, where the ambulacral 
vesicle is wanting, by projecting the fluid into the tube by a com- 
municating vessel. ‘The tubular part is thus distended and elongated, 
and again retracts itself by means of its muscular fibres, by which 
action the fluid is forced back into the interior. In progression the 
animal extends a few of its feet, attaches its suckers to the rocks or 
stones, then, by shortening its feet, it draws its body forward. The 
progression of the Asterias is thus very slow, and so regular that only 
the closest observation enables the spectator to discover the movement 
which produces it. Like the movements of the hands of a watch, the 
eye cannot quite follow it. When an obstacle presents itself—if, for 
example, a stone comes in its way—it raises one of the rays in order 
to obtain a point of support, then a second ray, and if necessary a 
third, and thus the animal creeps over the stone with as much ease 
as if it walked over the smooth sands. In the same way the animal 
creeps up perpendicular rocks, which is accomplished by means of 
these ambulacral feet and their suckers. Frédol says: “ If an Asterias 
is turned upon its back it will at first remain immovable, with its feet 
shut up. Soon, however, out come the feet like so many little 
feelers; it moves them backward and forward, as if feeling for 
the ground ; it soon inclines them towards the bottom of the vase, 
and fixes them one after the other. When it has a sufficient number 
attached the animal turns itself round. It is not impossible, whilst 
walking on the sea-shore, to have the pleasure of seeing one of these 
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