ECHINODERMA TA. 



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. rubens (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 Echinidae. The feet are thus muscular 

 fleshy cylinders, hollow in the centre, and very extensible ; by means 

 of them 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. Fredol 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 

 J 



