THE STARFISH. 269 



3. The slender and somewhat movable spines which 

 border the grooves, and which may be approximated to 

 cover the retracted tube feet. 



On the aboral surface note : - 



1. A low convex tubercle located between the center of 

 the disk and the junction of two of the arms. This is 

 the madreporite (named from its superficial likeness to 

 madrepore coral). Examine with a lens. 



2. At the tip of each arm a minute reddish spot, 

 the eye. 



3. Note again the arrangement of spines and branchial 

 tentacles. 



Internal Anatomy. Divide the crust along the lateral 

 margins and across the tip of the two arms which border 

 the madreporite between oral and aboral surfaces, being 

 very careful to cut no deeper than the crust. Continue 

 the cut entirely around the disk and across the bases of the 

 other arms. Then make a shallow, circular cut around 

 the madreporite, so that, when the loosened aboral crust is 

 lifted, this will remain in place. Then carefully lift the 

 crust, beginning at the end of an arm, looking beneath it 

 as it is lifted, and freeing it from all its attachments. 

 Observe that a pair of glandular organs (hepatic cceca) in 

 each arm lifts with the crust, being suspended from it 

 by membrane (mesentery). Along with these, and lying 

 above them, may be found also a pair of paler and more 

 granular reproductive organs. As the crust is lifted, 

 the hepatic caeca may be traced to their union in the base 

 of the arms before entering the stomach. The repro- 

 ductive organs may be traced to their (ovi- or sperm-) 

 ducts, which diverge on reaching the disk, each passing to 

 meet at its external aperture the duct of a similar organ 

 in the adjacent arm. As the crust of the disk is lifted, 

 there may be found the narrow terminal portion of the 



