76 ECHINODERMATA. 



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 grooves. Are they distinct? 



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

 they arranged? 



6. The openings to the bursa?, near the bases of the arms. 

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

 bursa, but Ophiura h^s 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 

 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. 



Grave: Ophiura brevispina I. Mem. Biol. Lab. Johns Hopkins Univ., 4, 



1900. Mem. Nat. Acad., 8, 1899. 

 : Ophiura brevispina II. An Embryological Contribution and Study 



of the Effect of Yolk Substance upon the Developmental Process. Jour. 



Morph., 27, 1916. 



ECHINOIDEA. 

 STRONGYLOCENTROTUS.i (Sea-Urchm.) 



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 

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



