226 ECHINODERMATA 



both extremities of a tooth and determine why the inner end 



is soft. 



Understand thoroughly how the jaws are used and why 

 the animal needs them. Why does the sea urchin not need 

 large hepatic caeca? 



Gemmill, J. F.: The Locomotor Function of the Lantern in Echinus 

 with Observations of the Allied Lantern Activities. Proc. Roy. Soc, 

 London, vol. No. 85, 1912. 



The Nervous System. — The nervous system is difficult to 

 demonstrate in dissections, but is easy to trace in sections. 

 It consists of: 



1. A nerve ring that encircles the esophagus at a point 

 just above the mouth. 



2. Five radial nerves that pass from the ring, along the 

 insides of the ambulacral areas of the test, to the ocular 



plates. 



The radial water tubes will be found in sections adjacent 

 to the radial nerves. The two are separated only by a nar- 

 row space, the pseudohemal canal. Between the radial 

 nerves and the tissue of the test there is another narrow 

 cavity, the epineural sinus. 



If time permits, students will find a dissection of the sand 

 dollar, Echinarachnius, valuable for purposes of comparison. 

 Special notes will not be necessary. Its shape and restricted 

 ambulacral areas should be studied in the light of its habits 

 and food supply. How does the animal move? 



Chadwick, H. C: Memoir No. 3, Echinus. Liverpool Marine Biol. 



Committee, 1900. 

 MacBride: Cambridge Natural History, Echinodermata. 

 Tennent: Variation in Echinoid Plutei. Jour. Exp. Zool., 9, 1910. 

 von Uexhull: Die Physiologie des Seeigelstachels. Zeit. f. Biol., 39. 

 : TJeber die Function der Polischen Blasen am Kauapparat der 



regularen Seeigel. Mitth. Zool. Stat. Neapel, 12, 1897. 



HOLOTHUROIDEA 



THYONE (Sea Cucumber) 



These animals may be found in protected and usually 

 muddy places, concealed in eel grass. They are generally 



