HARD PARTS OF THE SEA-URCHIN. 85 



(i.) The long, tapering, longitudinally fluted shaft. 



(ii.) The flattened, polished, toothed tip. 



(iii.) The ridge or column near the base, which served 

 for the attachment of muscles. 



(iv.) The highly-polished cup, by which each spine 

 articulates, in a ball and socket joint, with a polished tu- 

 bercle upon the surface of the shell. 



d. The ab-oral area, a small pentagonal region free from 

 spines, opposite the mouth. The centre of this area is 

 made up of four flattened, triangular, movable anal plates, 

 which meet in the centre and form the periproct. 



II. Remove the spines and soft parts, and after macerat- 

 ing the shell for a few days in a strong solution of caustic 

 potash, examine the hard calcareous corona, which remains 

 after all the soft parts have been dissolved. Examine first 

 the ab-oral surface, and notice : 



a. The nearly oval space which, in the fresh specimen, 

 is occupied by the four anal plates (Fig. 36, a). 



b. This space is bounded by the bases of the five trian- 

 gular ovarian plates (Fig. 36, b). These plates form a 

 five-rayed star, with a small perforation, the reproductive 

 opening, at the tip of each ray of the star. 



c. One of these plates is a little larger than the other 

 four ; rough, and covered with minute perforations. The 

 perforated portion is the madreporic body. 



d. A small triangular occular plate (Fig. 36, c) is 

 wedged into the angle between each two ovarian plates. 

 Each occular plate is perforated, near its outer edge, by 

 two very small openings. 



(If Strongylocentrotus is used for laboratory work in- 

 stead of Arbacia, considerable difference will be found in 

 the arrangement of these plates. The periproct contains a 

 great number of small plates in place of four large ones. 



