ECHINODERMATA 2OQ 



Ring canal: its position. Are there any other structures (sacs) in communi- 

 cation with the circum-oral ring-canal beside the stone canal and the radial water- 

 tubes? form and position? 



5. Nervous system. 



There is a radial nerve (in the skin) superficial to the radial water canal in each 

 arm. The radial nerves unite to form a circumoral nerve ring. 



6. Skeletal parts. 



Dried material and portions soaked for a day or so in a 10 per cent, solution of 

 potash should be used to supplement the alcoholic specimens. 



Is the skeleton complete, i.e., are the ossicles in contact? 



Are they similarly arranged on the aboral and oral surfaces? Which sur- 

 face shows the greater differentiation? Illustrate, and find a reason if you can. 

 How are the ossicles related to the spines? to the papulae? Study with some care 

 the ossicles forming the ambulacral groove, beginning at the middle line. 



Ambulacral rafters: shape and arrangement. 



Ambulacral pores; are they in, or between, the ossicles? 



Adambulacral ossicles (just lateral to the former); how do these compare in 

 number with the ambulacral ossicles? 



"Cross-shaped" ossicles. 



Which of the above bear spines? what kind? 



Place some of the cleaned ossicles in dilute hydrochloric acid. Result? What 

 is the significance of this result? 



7. Physiological experiments are possible only near the seashore. The animals 

 must be kept in sea water, and studied soon after being collected. When possible, 

 locomotion, the action of the ambulacral feet, feeding, and sensitiveness should be 

 studied. Do you find any indications, among the specimens provided, of the power 

 to renew a lost arm? With care and perseverance, at the proper time of year, the 

 sexual elements may be collected and the maturation, fertilization, and cleavage of 

 the ovum illustrated in this group. Teachers in inland schools should procure, 

 whenever possible, slides demonstrating the early development of the starfish or 

 sea-urchin. 



8. Compare briefly the external features of other "stars" with that already 

 studied. 



240. Sea-urchin (Echinus or Arbacia}. 



A few skeletons of sea-urchins and sand-dollars will be of great value in enabling 

 the pupil to see how the same general plan of structure may be varied, in different 

 organisms. 



1 . Spines (if present) : arrangement and method of attachment. Are they of 

 the same appearance and composition as the skeleton? Do you find any signs 

 of the former presence of ambulacral feet? If so what, and how arranged? Can 

 they all have the same function as in the starfish? Proofs? 



2. Ossicles; Make out the boundaries. Compare with the condition in the 

 starfish. What are the special advantages gained by each arrangement? Can 

 you find anything corresponding to ambulacral ossicles? (Look for the pores.) 

 What corresponds to the ambulacral groove in Asterias? Identify the interambu- 

 lacral ossicles. How arranged ? What is radial and what interradial in the urchin ? 

 What in the sea-urchin would correspond to the oral and aboral surfaces in the 

 starfish? Evidences? Find the madreporic body. Make a plot of all the os- 

 sicles in this region, noting the differences. Find the genital pores. 



14 



