226 



ZOOLOGY 



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 



Fig. 131. — Diagram of a 



1. Epidermis. 



2. Mesoderm. 



3. Perihaemal space in the skin. 



4. Peritoneal lining of body-cavity. 



5. A branchia. 



6. Paired caeca from intestine. 



7. Mesentery supporting caeca. 



8. Spine. 



9. Ossicle in skin. 



10. Pedicellaria. 



11. Ambulacral ossicle. 



section of the arm of a Starfish. 



12. Adambulacral ossicle. 



13. Eadial trunk of water - vascular 

 system. 



Badial trunk of blood - vascular 



system of Ludwig. 

 Eadial nerve connected with plexus 



under epidermis. 



16. Ampulla of tube-foot. 



17. Tube-foot. 



18. Perihaemal space. 



19. Coelom. 



U. 



15 



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 



