226 AOQOROGN4 
a single ossicle, which supports a terminal tentacle bearing a 
number of pits of pigmented cells, called collectively the eye- 
spot. Between this single ossicle and the other ambulacral 
Fic. 181.—Diagram of a transverse section of the arm of a Starfish. 
1, Epidermis. 12. Adambulacral ossicle. 
2. Mesoderm. 13, Radial trunk of water - vascular 
3. Perihaemal space in the skin. system. 
4, Peritoneal lining of body-cavity. 14. Radial trunk of blood vascular 
5. A branchia. system of Ludwig. 
6. Paired caeca from intestine. 15. Radial nerve connected with plexus 
7. Mesentery supporting caeca. under epidermis. 
8. Spine. 16. Ampulla of tube-foot. 
9. Ossicle in skin. 17. Tube-foot. 
10. Pedicellaria. 18. Perihaemal space. 
11. Ambulacral ossicle. 19. Coelom. 
plates all the new plates appear. The tentacle at the tip of the 
arm, together with the eye-spot, is surrounded by a circlet of 
spines. 
On the dorsal surface of both disk and arms numerous 
spines are scattered, and amongst them many pedicellariae 
(Fig. 131). These must be regarded as modified spines; they 
consist of a basilar plate and of two blades which snap against 
one another like the two limbs of a pair of forceps—in some of 
them the blades cross one another as they do in a pair of 
scissors. The function of these pedicellariae seems to be to 
catch hold of foreign bodies, and so keep parasites from settling 
