ECHINUS. 149 



elongated or retracted, at the pleasure of the animal, by a 

 very curious mechanism, which I shall presently describe. 

 By bending them on either side, in their expanded state, 

 the Asterias is capable of effecting a slow progressive mo- 

 tion; so that these processes may be regarded as correspond- 

 ing to feet, being levers for the advance of the body. This, 

 it may be remarked, is the first time that we meet with or- 

 gans of that description in our progress through the animal 

 kingdom. Each of these feet is terminated by a concave 

 disk, which when applied to any flat surface acts as a sucker, 

 on the principles already adverted to.'' Reaumur counted 304 

 of these feet in each of the five rays of the star fish, making 

 1520 in all.t Each foot consists of a tube, closed at the 

 outer end, and the stem of which, after passing through the 

 aperture in the integument, is dilated into a bag or reservoir 

 of fluid; as is shown in Fig. 97. ^ By the contraction of this 

 reservoir, the fluid it contains is propelled into the outer 

 portion of the tube, which protrudes by being thus distend- 

 ed; the foot fixes itself, by means of its terminal fleshy disk, 

 to the point it touches, and then, by retracting, draws the 

 body along for a short distance. By the retreat of the fluid 

 into its reservoir^ the foot is again detached, and ready 

 to be moved forwards, and is thus made instrumental in 

 taking another step, by a repetition of the same process.:}: 

 From the shortness of these feet, notwithstanding their great 



• Page 105. 



\ !Memoires de rAcadcmIc des Sciences, 1710, p. 487. 



\ The mechanism by which the feet arc protruded and retracted is illus- 

 trated by the diagram. Fig". 97, which exhibits the bladders connected with 

 them, in different states of distention and contraction. Fig 96 shows the 

 upper side of the umbulacra, and of the bladders comiected with the feet. 

 Dr. Grant, from some observations which he made on the structure of the cilia 

 of the Beroe pileus, is led to suspect that the rapid vibrations of these sin- 

 gular organs in the lowest animals may depend on the undulations of water 

 conveyed through elastic tubes along their bases, in a manner resembling 

 the injection of the tubular tentacula of Actinix and Astcriae. If this con- 

 jecture were verified, he remarks, one of the most remarkable phenomena 

 of animal motion, though one of the most frequent, would lose much of its 

 present marvellous character. 



