SEA-URCHINS 457 



edges, constitute a firm protective "test". The base of each 

 spine is hollowed out into a cup which fits over a corresponding 

 knob on the test, so that there is a ball-and-socket joint arrange- 

 ment, and by means of special strands of muscle the spine can be 

 moved in any direction. This is an adaptation to locomotion as 

 well as to protection. Effective tube-feet are, however, present, 

 and in a living sea-urchin these may be seen protruding between 

 the spines, and if we compare the animal to a globe with upper 

 and lower poles, their distribution is expressed by saying that they- 

 are arranged in five meridional bands. 



As in a star-fish, the mouth is situated in the centre of the 

 under surface, but in this case there are five pointed structures 

 to be seen projecting from it, these being the tips of hard jaws 

 shaped something like the front teeth of a rabbit. The opening of 

 the intestine is placed almost in the middle of the upper surface. 



On scraping away the spines, the characters of the test may be 

 studied in detail, and it is found to be made up of twenty regularly- 

 arranged sets of plates arranged in ten strips, which are broad in 

 the middle but narrow at either end, like the " gores " which make 

 up a balloon, or are sewn together to cover a child's cloth-covered 

 ball. Two meridional rows of plates are united to make up each 

 of these bands. Five of these differ in one important particular 

 from the others which alternate with them, the distinction resting 

 in the presence of a series of pores arranged in pairs, and placed 

 near either edge of the band to which they belong. Each pair 

 of these pores belongs to a tube-foot, which is thereby placed in 

 communication with a radial water-vascular vessel, and with an 

 ampulla (see p. 453), both of these being situated within the test. 

 The presence of these pores enables us to distinguish between five 

 ambulacral areas, bearing tube -feet, and five intervening inter- 

 ambulacral areas. The former correspond to the radii about which 

 the body is symmetrically built, and the others are consequently 

 interradial. It may be useful to institute a comparison with the 

 star-fish, taking for the purpose, not the common form, but the kind 

 {Goniaster) shaped like a pentagonal disc. If we suppose the 

 body of such a form to gradually swell so as to become spheroidal, 

 while at the same time the original upper surface gets smaller 

 and smaller, the lower surface (bearing the tube-feet), becoming 

 correspondingly extended, the sea-urchin type will be produced. 

 For the five ambulacral grooves will be converted into the five 



