

FIG. 69. -THE COMMON SEA-URCHIN, Echinus esculentus. THE SPINES 



HAVE BEEN RUBBED OFF FROM HALF OF THE SHELL OR TEST. 



As a preliminary discipline in exactness, a thorough study of the skeletal 

 parts of the sea-urchin is recommended to the student. 



It is a mespdermic skeleton, covered during life by a delicate, tissue- 

 paper-like, ciliated ectoderm. 



At the lower pole is the mouth, from which project the five teeth of 

 Aristotle's lantern a remarkable apparatus used in mastication and also 

 in locomotion on a flat surface. 



At the upper pole the food-canal ends and around the polar area is a 

 complicated apical disc with five genital plates and five so-called ocular 

 plates. Through a hole in each ocular plate a sensitive tube-foot emerges. 

 Through a hole in each genital plate the germ-cells are shed into the water. 

 But the largest of the genital plates, called the madreporic plate, has a 

 structure like the rose of a watering-can, and serves as the entrance to the 

 water-vascular system which enables the sea-urchin to climb up the side 

 of a shore-pool. 



In a line with the ocular plates ai*e five narrow areas, each composed of 

 a double row of plates. These are called ambulacra! areas, and along them, 

 through numerous pores, the locomotor suctorial tube-feet of the water- 

 vascular system emerge. There are also spines on these ambulacral areas. 



In aline with the genital plates are five broad areas, each composed of a 

 double row of plates, bearing spines only. These are called inter-ambu- 

 lacral areas. 



The spines are of various sizes. They are longitudinally grooved and 

 have a beautiful internal zoned architecture. They are worked by muscles 

 on ball and socket joints. The ball is a prominence on the test ; the 

 socket is the base of the spine. Among the ordinary spines are four kinds 

 of small snapping pedicellariae and also spherical sensory sphaeridia. 



244 



