OPHIUBOIDEA. 67 



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- 

 tially from starfish in their method of locomotion and their 

 method of feeding. 



Examine a specimen and notice : 



1. The appearance of the disk and arms. Are the spines 

 similar to those of Asterias? The arms are more flexible. In 

 what direction do they bend easiest ? 



2. The five buccal plates, one of which bears a madreporic 

 opening that is not easily seen. 



3. The size and shape of the mouth. 



4. The ambulacral groove. Is it distinct? 



5. The ambulacral feet. Do they have suckers? How are 

 they arranged? 



6. The openings to the bursse, near the bases of the arms. 

 Most Ophiurians have five pairs of these openings, one for each 

 bursa, but Ophiura has ten pairs, two for each bursa. 



Draw an oral view of a specimen. 



Place a living specimen in a dish of sea-water and watch its 

 movements. 



1. Compare the rate and method of movement with Asterias. 



2. Are all of the arms used in progressing in the same way? 



3. See if the arms can be used interchangeably or if a cer- 

 tain one is always directed forward. 



4. Are the ambulacral feet of any service? Do they adhere? 

 The internal structure shows that the stomach is not eversible 

 and that the hepatic caeca do not extend into the arms. Is 

 there any correlation between these two facts? 



The nervous and water-vascular systems are very similar 

 to those of Asterias, but here the former lies within instead of 



