ECHINODERMS 451 



or Worm, the body can be divided into corresponding halves in one 

 way only, i.e. by a median plane which separates the right side 

 from the left, but here this can be done in five different ways. 

 Another way of putting it is to say that the body is regularly built 

 up around a series of axes which radiate from a common central 

 point. Any part or organ which is cut by one of these axes may 

 be said to occupy a radial position, while interradial structures 



A. 



TUBE-FOOT 



PEDICELLARIA 



Fig. 277. Common Star-Fish ( Uraster rubens) 

 A and B, Upper and under surfaces, reduced ; c and D, much enlarged. 



fall between adjacent axes. There is, it is true, a distinction 

 between upper and lower surfaces, as in vertebrates, &c., but it 

 is by no means clear that we are justified in considering these to 

 be truly dorsal and ventral. 



Turning our attention to the pale under surface, we shall 

 see, in its centre, the mouth, entirely devoid of jaws and placed 

 in the middle of a soft area from which a deep groove runs along 

 each arm to its tip. Occupying these five grooves are a large 

 number of slender cylindrical structures which observation of a 

 living star-fish shows to be used in locomotion, and which have 

 consequently been termed tube-feet. Each groove has fancifully 

 been termed an ambulacrum^ because, together with its tube-feet, 

 it suggested to the giver of the name a little pleasure grove or 

 tree-lined avenue (Lat. ambulacrum]. The tips of the arms are 

 commonly seen to curl upwards, and each of them bears a reddish 

 eye-spot. 



