116 LABORATORY WORK. 



fish? Between each two sets of ambulacra! plates are 

 found the larger interambulacral plates Which plates, 

 ambulacral or interambulacral, bear rounded prominences 

 for the articulation of the spines? Making a comparison 

 with a starfish , where would you draw the line between two 

 rays of the sea-urchin? Illustrate by a sketch. 



Follow a ray from the oral area to the pole opposite the 

 mouth. Notice in the centre of this pole a circular anal 

 area, made up of small anal plates. How many plates 

 make up the boundary of this circle? Examine them 

 under the lens and decide which one compares in structure 

 with the madreporite of the starfish. Is it radial or inter- 

 radial in position? HOW T many of these plates bear small 

 pores? Sketch this region, showing the anal area and the 

 tips of the rays, and label the parts, deciding which of the 

 perforated plates must be genital and which must be 

 ocular plates by comparing with their relative position, 

 radial or interradial, in the starfish. With what is the 

 madreporite associated? What parts must belong to the 

 aboral surface of the starfish? 



Draw, in outline, a starfish, marking on it the position 

 of the madreporite, genital openings, ambulacral and 

 interambulacral plates, radial canals, and eye-spots. Cut 

 this out and illustrate by bending the rays aborally how 

 this starfish can be made to resemble the sea-urchin. 



INTERNAL STRUCTURE. 



Open an alcoholic urchin by breaking into the equator 

 of the test and then continue the opening by breaking, 

 bit by bit, with the forceps around the shell, taking care 

 that the fleshy parts beneath be not injured. Continue 

 until the whole of the aboral surface is removed, leaving 

 all the soft portions in the oral half of the test. 



