STRUCTURE OF THE STARFISH. 59 



horizontal plate, the inter-airihulacrdl ossicle, runs out- 

 wards and downwards, and forms part of the outer skele- 

 ton of the ray. On the lower or outer surface of each 

 inter-ambulacral ossicle two slender spines are articulated 

 by movable joints at their bases. 



4. Running outwards and upwards from the outer ends 

 of the inter-ambulacral ossicles are much larger and thicker 

 plates, each of which canies three or four thick club- 

 shaped movaljle spines. Each of these plates articulates 

 with several (three or four) of the inter-ambulacral 

 plates. 



5. The remainder of the wall of the ray is made up of 

 a membrane which contains an irregular network of ossi- 

 cles with immovable spines. 



i. !Make a sketch of the section, showing all these 

 points. 



J. Cut off the ab-oral wall of the ray which has been 

 removed, and clean off the dried remains of the soft parts, 

 in order to expose the inner surfaces of the ambulacral 

 plates. The cleaning will be more easily done after the 

 rav has been soaked in warm water Ions: enough to 

 soften it. 



1. Xotice the vertebral ridge; a longitudinal elevation 

 along the middle line of the floor of the ray. The ridge 

 is formed by the union of the upper ends of the ambu- 

 lacral ossicle, and a shallow longitudinal furrow or suture 

 marks the line where those of opposite sides meet. The 

 vertebral ridge is also marked by hundreds of fine parallel 

 transverse fissures, the sutures between adjacent ambu- 

 lacral plates. These fissures give the ridge a resemblance 

 to the vertebral column of a vertebrate. In a ray which 

 has been softened in water it will be seen that there is con- 

 siderable power of motion between the ambulacral plates 



