STARFISH. Ill 



surface and draw a line through it dividing it into sym- 

 metrical halves. How many such lines can be drawn? 

 The arm opposite the madreporite is known as the ante- 

 rior ray* 



With the needle demonstrate that the calcareous plates 

 are not on the outside. What covers them? Are the 

 spines movable on the plates? Scattered over the aboral 

 surface are numbers of fleshy, finger-like projections, the 

 branchice. Look at the very tip of the arm and find the 

 rounded red eye-spot (recognized with difficulty in pre- 

 served material). 



INTERNAL STRUCTURE. 



Cut into the side of one of the arms, carrying the inci- 

 sion outward to near the tip, crossing to the opposite side 

 and then back towards but not quite to the disc. Fold 

 back the flap thus separated and notice the following 

 structures : 



Attached to the aboral surface the lobular hepatic cceca, 

 each supported by a thin membrane (mesentery). 



On the floor (oral surface) a series of thin-walled vesicles, 

 the ampulla. By means of a needle ascertain if these am- 

 pullae are connected with the ambulacra. 



Continue the removal of the aboral surface from the 

 rest of the body, taking care that all soft parts, including 

 the hepatic caeca, are separated from it and left in the 

 oral portion, that the portion immediately around the 

 madreporite be left intact, and that one arm be left 



* The reasons why this is called anterior rather than posterior 

 cannot be worked out on the forms selected for dissection, but can 

 only be seen by a comparison with the cake-urchins (Clypeas- 

 troids) and heart-urchins (Spatangoids). 



