THE ECHINODERMATA 261 



an affinity to crinoids, but there are no stalk and no arms. 

 The skeleton (or test) has somewhat the appearance of a 

 globe with poles flattened, but one (the oral pole) so much 

 more so than the other (aboral) that the flattened surface 

 is but little way below the equator (or ambitus). A full- 

 grown specimen is about 36 mm. in diameter at the ambitus 

 and 25 mm. high. Sometimes the fossil is found with 

 the movable spines (or radicles) which characterize the 

 sea-urchins still in their natural position ; but as a rule 

 they fell off before burial, and the test only shows the 

 tubercles with which they were articulated. Some of 

 these are large and prominent, and carried thick cylin- 

 drical radicles, up to 95 mm. in length. These large (or 

 primary) tubercles are arranged in five double rows, 

 between which are narrower areas each with a double 

 row of much smaller tubercles. The difference between 

 these alternate areas is most striking near the aboral 

 pole, and becomes much less so near the oral pole by the 

 increase in width of the narrower areas and in the size 

 of their tubercles. But there is a more important dis- 

 tinction that persists throughout : the narrower areas 

 carry a series of fine pores, always arranged in pairs, and 

 in each area there are two vertical columns of pore-pairs. 

 These pores, in life, transmitted finger-like muscular 

 tubes kept tense by the pressure of water in them from 

 an internal system of water- vessels (analogous to blood- 

 vessels) : these are the tube-feet (or podia), the organs 

 of locomotion of sea-urchins. In life they extend out 

 beyond the long radicles, and their sucker-like ends can 

 adhere firmly to any foreign body. 



