ECHINOIDEA. 



105 



In one great group of the Echinoids, the ambulacral areas 

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

 (fig. 58), when they are said to be " perfect" (ambulacra per- 

 fecta) or " simple." In another great group the ambulacral 



Fig. t$.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. 



areas are not thus continuous from pole to pole, but simply 

 form a kind of rosette upon the upper surface of the shell 

 (fig. 59). In these cases as in the common Heart-urchins 



Fig. 59. Scutella subrotunda, showing petaloid ambulacra. Miocene. 



the ambulacral zones are said to be "circumscript" (ambulacra 

 circumscriptd) or " petaloid." 



The most important external structures of the corona are 

 the tubercles and spines. The tubercles are rounded eleva- 

 tions upon which the spines are carried (fig. 60). They vary 

 much in their dimensions, and receive special names accord- 

 ing to their size or position on the test. Ordinarily the tuber- 

 cle consists of a rounded ball or hemisphere (the " mamelon ") 

 supported upon a conical process (the "boss") which arises 

 from the plate. The ball of the tubercle may or may not be 

 perforated for the insertion of a ligament which is attached to 

 the articular surface of the spine. In many cases (as in fig. 

 60) the base of the tubercle is surrounded a round or oval 

 smooth and excavated space which is termed the " areola." 



The spines are movable appendages which are jointed 



