150 INVERTEBRATE ZOOLOGY 
ambulacral suckers which surround the mouth, and near them the 
five groups of pincer-like pedicellarie. Observe the long slender 
stalks of these organs. Near the margin of the peristome are 
five groups of ambulacral feet. 
Exercise 1. Make a drawing of the oral surface on a scale of 2. 
With forceps remove some of the pedicellaria, mount them, 
and study them under the microscope. Note the three minute 
jaws and the long stalk. Press on the cover-glass and cause 
the jaws to open and shut. 
Exercise 2. Make a drawing of a pedicellaria. 
Study the structure and method of articulation of the spines. 
Pull off several and notice their ball and socket joint, also the 
delicate muscles by which they are moved. Notice the fluting 
of the shaft. 
Exercise 3. Make a semidiagrammatic drawing of a spine on a 
large scale showing the articulation and the muscles. 
Remove the spines from the dried specimen and thoroughly 
clean the shell. This is accomplished the most effectually by 
placing it in a strong solution of warm caustic potash for a 
short time. Great care should be taken, however, not to leave 
it in the solution too long or it will fall to pieces. Study the 
aboral side of the shell. Observe the rows of tubercles on 
which the spines have articulated, also the bands of minute 
holes, the ambulacral pores, through which the ambulacral feet 
have projected. There are ten of these bands arranged in 
pairs, and each pair represents an ambulacral area or a ray. 
Between the five rays are the five interambulacral areas or the 
interrays, which are somewhat broader than the rays; count 
the rays and the interrays. 
The center of the aboral surface is free from spines and is 
made up of several small plates. It is called the periproct and 
