Branch Echinodermata 41 



animal can be greatly lengthened, and the disk-like tip 

 by means of which it holds on to objects. Make a 

 drawing showing the structure of the sucker-like tip. 

 The flat central disk is supported by a calcareous plate. 



IV. — In addition to the numerous ambulacra on the outer 

 margin of the peristome, find five pairs with somewhat 

 larger tips surrounding the mouth. See between the 

 pairs of ambulacra the groups of pedicellariae, appearing 

 as miniature forceps on slender stalks. Are these 

 present on other parts of the body ? 



V. — Find other ambulacra among the spines over the rest 



of the body. How generally are they distributed .■' 



VI. — Extract at its base one of the long spines with which 

 the body is covered. Study it with a hand lens and 

 make a careful drawing showing the following parts : — 

 a. The basal piece extracted from the body wall. 



d. The membranous part by which the spine is attached. 



c. The long spine proper, with its tapering surface 

 fluted longitudinally. 



d. The flattened tip, with its polished tooth-like 

 surface. 



VII. — Dissect out the flexible basal part of the spine to see 

 the ball and socket joint. Make a diagrammatic sketch 

 showing the structure accurately. 



VIII. — Describe the arrangement of the spines with ref- 

 erence to the mouth, noting especially the differences in 

 the length and the structure of the tips. 



IX. — Make a careful drawing of the spineless aboral area 

 on the side opposite from the mouth. Show the four 

 anal plates ; these form the periproct. 



X. — Study now a specimen on which the spines and soft 



parts have been remoyed by soaking for a few days in 

 a strong solution of caustic potash. On the aboral sur- 



