144 INVERTEBRATE ZOOLOGY 
Cut off an arm of the dried specimen, and also a bivial arm 
of the fresh one, and examine the cut surface of each. The 
edge of the calcareous plates will be seen, as well as the spaces 
between them. Notice the slender plates which form the sides 
of the ambulacral groove ; also just beneath the median ridge, in 
the upper part of the apex of the groove, a minute opening. 
This opening is the radial canal, which extends the length of the 
arm; its function will be explained when the ambulacral system 
is described. If a portion of the arm be soaked for a short time 
in a strong solution of warm caustic potash, the soft parts will 
be destroyed and the plates will be seen more distinctly. Care 
should be taken not to allow the potash to act too long or the 
arm will fall to pieces. 
Exercise 4. Make a sketch of the cut edge of the arm on a scale 
of 8, showing the edges of the plates and the radial canal. 
Cut off the aboral wall of the severed arm of the dried 
specimen and scrape away the remains of the internal organs 
and the ambulacral feet. Study the inner surface of the 
ambulacral groove. Note the two rows of slender transverse 
plates which form the sides of the groove, and on each side 
between every two plates, the minute ambulacral pore. 
Bxercise 5. Make a drawing on a scale of 3 of the inner surface 
of the ambulacral groove, showing the plates and the pores. 
Cut off the aboral wall of the central disc of the dried speci- 
men, scrape away the remains of the internal organs, and study 
the arrangement of the plates in the inner surface of the oral 
body-wall. Note the circular mouth protected by converging 
spines, also the membranous peristome. Observe the con- 
vergence of the five arms about the peristome; also the inter- 
radial partitions which separate the base of the arms. 
Exercise 6. Make a life-size drawing showing these features. 
