ARBACIA. 61 



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. Do you find a madreporic plate ? 



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 ? 



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 cseca 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 

 on the surface of the arm, the entire arm being encased with four 

 or more rows of shields. They can be studied best in sections. 



ECHINOIDEA. 



ARBACIA. 1 (Sea-Urchin.) 



In some localities sea-urchins can be found in tide pools 

 or near low-tide mark, where they may be -very abundant. In 

 other localities they can be obtained only by dredging. When 

 possible they should be observed in their native places and the 

 conditions noted. 



1 These directions will serve for any of our common sea-urchins. 



