ECHINODERMATA. 263 



and disposition of which serve to characterise the genera and the 

 species. 



Another species, Asterias aurantiaca, will give an exact idea of the 

 general type of animals of this order. This zoophyte, which is repre- 

 sented in Fig. 106, is common in the northern seas ; it has five rather 

 long arms, furnished with spines which are of an orange colour hence 

 its name. When we see one of these animals stranded upon the 

 shore, it appears to be entirely destitute of all power of progression. 

 But the star-fish is not always immovable ; it is provided with an 

 apparatus for locomotion, which appears to serve at the same time the 

 purposes of respiration ; for nature is not sparing in her gifts to 

 the least organized beings ; she bestows upon them feet, with respi- 

 ratory organs, or lungs, which have the power of locomotion. 



The muscular system, as already stated, is almost always present 

 in the Echinodermata, but the organs of locomotion are very various, 

 the principal being the membranous tubes usually termed feet, or 

 ambulacra, which issue from the ambulacral apertures ; but besides 

 these, the rays themselves are movable, and in animals which are free 

 to move from place to place these are used, for the purpose. Thus in 

 the common star-fish the rays may be bent towards the upper or lower 

 surface of the disk, so as to facilitate its advance either in water over 

 small spaces or up the vertical face of rocks. These ambulacra are 

 very numerous, disposed in rows along the under surface of the rays ; 

 thus in A. aurantiaca there are two simple rows of 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 on each ray, and 

 each foot has an undivided vesicle. 



Each of these ambulacra consists of two parts, an internal and 

 generally vesicular 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 terminat- 

 ing in a sucker, usually in the form of a disk slightly depressed in the 

 centre. 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 

 vesicle, which forces the fluid into the hollow tube, or, where the 

 vesicle is wanting, by projecting the fluid into the tube by a com- 

 municating vessel. The tubular part is thus distended and elongated, 



