150 INVERTEBRATE ZOOLOGY 



ambulacral suckers which surround the mouth, and near them the 

 five groups of pincer-like pedicellariae. 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 pedicellariae, 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 



