HARD PARTS OF THE SEA-URCHIN. 89 



definite, nearly straight suture, just outside the area of 

 ambulacral pores. Each inter-ambulacral plate is nearly 

 rectangular, the sides being longer than the ends. It 

 articulates by its sides with the adjacent inter-ambulacral 

 plates of the same series ; by one of its ends with the am- 

 bulacral plates of the same ray ; and by the other end, 

 which is pointed, with the ends of inter-ambulacral plates 

 of the adjacent ray. On the outer surface of each inter- 

 ambulacral plate are from one to four tubercles for the 

 attachment of spines. 



I. Draw a group of inter-ambulacral plates, showing 

 the manner in which they unite with all the plates which 

 touch them. 



m. Break up a small portion of an ambulacral area, 

 observing that the plates of the two sides of the ray 

 separate readily along the ambulacral suture, while those 

 on the same side of the ray separate from each other much 

 less readily along sutures which are perpendicular to and 

 opposite the angles of the ambulacral suture. Notice : 



1. That each plate is similar in shape to an inter-ambu- 

 lacral plate. They are so placed that their flat ends artic- 

 ulate with the flat ends of the inter-ambulacral plates, and 

 their pointed ends with the ambulacral plates of the other 

 half of the ray. 



2. On the outer surface of each plate, near the pointed 

 end, a tubercle for the attachment of a spine. 



3. The opposite end, the end farthest from the middle 

 of the ray, is perforated by three or four pairs of am- 

 bulacral pores. The two pores which form a pair are 

 inclined towards each other, so that their external ends 

 are much closer together than their internal ends. The 

 one nearest the middle of the ray serves for the pas- 

 sage of the tube which joins the foot to the radial water- 



