166 



THE SEA SHORE 



that it is wonderfully constructed. Even the spines, which are 

 in themselves uninteresting objects to the naked eye, are most 

 beautifully formed, a transverse section revealing a radiate or 

 reticulated structure when viewed through the microscope. Each 

 spine has a concave base which fits on a little tubercle of the 

 calcareous shell or test that covers the body of the animal, forming 

 a perfect ball-and-socket joint, and is capable of being moved in 

 any direction by means of small muscular bands. 



On removing the spines the shell is seen to completely enclose 

 the animal with the exception of the mouth, with its masticatory 



apparatus, and the 

 small area around it 

 which is covered by 

 the uncalcified skin 

 just referred to. 



FIG. 109. SEA URCHIN WITH SPINES REMOVED 

 ON ONE SIDE 



FIG. 110. APEX OF 

 SHELL OF SEA 

 URCHIN 



At the very top 

 of the shell, exactly 

 opposite the mouth, 

 there is a small plate 

 perforated by the ex- 

 tremity of the digestive tube. Bound this are five angular plates, 

 each perforated by the ducts of the ovaries or egg-producing 

 glands, but one of these is enlarged and further perforated, that it 

 may serve the second purpose of allowing water to enter the system 

 of water tubes that supply the tube-feet, and thus corresponds 

 exactly with the plate already noticed on the upper surface of the 

 starfish. Between these are five smaller plates, each with a rudi- 

 mentary eye that receives a fine nerve-thread. 



The remaining and greater portion of the shell of the urchin is 

 composed of ten radiating segments, each of which is made up of 



