I20 PHYLUM ECHINODERMATA 



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 joints, 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 of 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 solution of warm caustic potash for a short time. 

 Great care should be taken, however, not to leave it in the solu- 

 tion 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 ; note 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 contains 

 in its center the minute anus. Surrounding the periproct are ten 

 plates, which also bear no spines. Five of these, which are larger 

 than the others, are situated at the ends of the interrays and are 

 pierced each by a small hole. These plates are called the genital 

 plates, and the holes are the external openings of the genital 

 organs. One of the genital plates is larger than the others and is 

 porous ; it is the madreporite. The five smaller of the ten plates 

 which surround the periproct are situated at the ends of the rays. 

 They are called the ocular plates. Each is pierced by one or two 



