Fig. 110.—Apex of Shell of Sea Urchin
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.
At the very top of the shell, exactly opposite the mouth, there is a small plate perforated by the extremity of the digestive tube. Round 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 rudimentary 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 two rows of flat angular plates united at their edges. Five of these segments, arranged alternately with the others, are perforated by numerous holes, through which the tube-feet of the urchin are protruded, while the remainder are imperforate; and all ten plates bear the little hemispherical processes to which the spines are jointed.
Fig. 111.—Shell of Sea Urchin with Teeth protruding
One of the most interesting features of this urchin is undoubtedly its complex and wonderful masticating system. There are five teeth, symmetrically arranged, and all pointing towards the centre of the mouth. Each is attached to a wedge-shape jaw, made up of several pieces, and the whole apparatus is attached by ligaments to loops in the interior of the shell, and is moved by no less than thirty distinct muscles. The complete system may be readily dissected out, and is well worthy of study and preservation. (The harder portions of the system may often be found in the interior of the empty shell of an urchin after the softer structures of the body have decayed away.)