228 



ANNULOIDA. 



zones are called the " interambulacral areas." The remaining 

 five zones alternate with the former, and are composed of 

 very much smaller plates, which are perforated by minute 

 apertures or pores. Through these apertures are emitted the 

 little suctorial tubes of the water-vascular system the so- 

 called " ambulacral tubes " or " tube - feet " by means of 

 which the animal moves. Hence these zones of perforated 

 plates are termed the " ambulacral areas " or " poriferous 

 zones." 



In one great group of the Echinoids the ambulacral areas 

 pass from the centre of the base of the shell to its summit 

 (figs. 117, A, and 119), when they are said to be "perfect" 

 (ambulacra perfecta) or " simple." In another great group 



Fig. 119. Galerites albogalerus. The first figure shows the under surface with the mouth 

 and anus ; the middle figure is a side view ; and the right-hand figure shows the upper 

 surface, with the ambulacral areas converging to the apical disc. White Chalk. 



the ambulacral areas are not thus continuous from pole to 

 pole, but simply form a kind of rosette upon the upper 

 surface of the shell (figs. 117, D, and 120). In these cases 



Fig. 120. Scutella subrotunda, showing petaloid ambulacra. Miocene. 



as in the common Heart-urchins the ambulacral zones 

 are said to be " circurnscript " (ambulacra circumscripta) or 

 " petaloid." 



The most important external structures of the corona are 



