60 ECHINODERMATA. 



It consists of a nerve ring which encircles the mouth and Ues 

 just ventral to the circular water canal, and five radial nerves 

 that extend down the arms just beneath the radial water tubes, 

 to end at the tips of the arms in pigment spots, the eye- 

 spots. The whole central nervous system is superficial and forms 

 a portion of the outer covering of the body. The radial 

 nerves can be seen by separating the rows of ambulacral feet, 

 but it is much more satisfactory to study them in prepared 

 sections. 



Muscular System. — Examine the walls of the starfish for 

 its muscular system. If time permits, it will be desirable to 

 macerate a portion of an arm to see. the skeleton to which these 

 muscles are attached. 



Study prepared sections of the arm of a small starfish and 

 determine the relation of organs. 



1. The hepatic caeca. How are they supported? What 

 is their structure? 



2. The radial canal, connecting tubes, tube feet, and ampullse. 



3. The thickened, deeply stained, radial nerve between the 

 tube feet and below the radial water tube. 



4. The perihsemal canal, divided by a thin partition, that lies 

 between the radial water tube and the radial nerve. 



Make a draiving of a section of an arm that will show these 

 points. 



Understand how a starfish can open an oyster or a mussel 

 and how it digests it when open. How can it digest a barnacle 

 or a small snail ? How does it respire? 



OPHIUROIDEA. 



OPHIURA. (Serpent-Star.) 



These animals live more or less concealed in crevices, shells, 

 eel-grass, etc., and may be obtained either by dredging or by 

 pulling a dip-net through eel-grass. They are not conspicuous 

 objects along the shore, as are starfish, and they differ essen- 



