88 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



ovarian plate to the corresponding angle of the mouth- 

 pentagon. These sutures divide the corona into five areas 

 or rays (Figs. 36 and 37, 1, 2, 3, 4, 5), the broad ends 

 of which abut upon the mouth-pentagon and form its 

 sides, as in the starfish, while the tip of each ray lies be- 

 tween two of the ovarian plates, and ends with an occular 

 plate. 



f. Mark three of the inter-radial sutures with ink, and 

 notice a second time the points mentioned in e. 



g. Observe the double row of convex, highly-polished, 

 alternating tubercles for the attachment of spines, along 

 the middle of each ray (Figs. 36 and 37, e, e, e, e). No- 

 tice that these doul)le rays run from an occular plate to the 

 notch in the middle of the corresponding side of the mouth- 

 pentagon. 



h. In the middle of each double row a zigzag amhu- 

 lacral suture (Figs. 36 and ?>T •>/,/, f,f)- This line also 

 runs from an occular plate to the middle of a side of the 

 mouth-pentagon. It corresponds to the suture along the 

 vertebral ridge of a starfish, and is formed by the meet- 

 ing of the two sets of ambulacral ossicles which constitute 

 the ambulacral area in the middle of each ray. 



i. On each side of the double row of tubercles a double 

 row of ambulacral pores (Figs. 36 and 37, g, g, g, g). 



J. Outside the ambulacral area a row of inter-ambulacral 

 plates (Figs. 36 and 37, h, h, h, h). The outer ends of 

 the inter-ambulacral plates on opposite sides of adjacent 

 rays meet to form the inter-radial suture. 



k. Break a corona to pieces by pulling upon opposite 

 edges of the mouth-pentagon. Notice that the plates 

 separate most readily along the inter-radial sutures. 

 Break off some of the inter-ambulacral plates, and notice 

 that they separate from the ambulacral plates along a 



