348 



ECHINODERMATA 



[CH. 



moved in any direction. The skin covering the sheath has de- 

 veloped a specially thick nervous layer. 



Sea-urchins such as we have been describing live on stony or 

 rocky bottoms, over which they slowly creep by means of their 

 tube-feet. The spines are pressed against the substratum and 

 keep the animal from rolling over under the pull of the tube- 

 feet and also help to push it on. The spines are usually of two 

 distinct sizes, longer primary spines, and shorter secondary 



FIG. 163. Diagrammatic vertical section of a Sea-urchin, 

 after Hamann. 



From Leuckart, 



1. 



Mouth. 2. Intestine cut short. 3. Siphon. 4. Eectum. 5. Anus. 

 6. Ventral "blood "-vessel on intestine. V. Dorsal "blood "-vessel 



on intestine. 8. Stone-canal. 9. Madreporic plate. 10. Genital 

 rachis. 11. Water-vascular ring. 12. Nerve-ring. 13. Tube-foot 

 with ampulla. 14. Eadial nerve. 15. Eadial water-vessel. 16. Polian 

 vesicle. 17. Muscles; those on the left pull Aristotle's Lantern out- 



wards, those on the right retract it. 18. Ocular plate. 



spines. The forest of spines has a kind of undergrowth of 

 pedicellariae. These are of several kinds and are much more 

 highly finished organs than those of star-fishes ; they have a long 

 stalk, which is partly stiffened by a delicate calcareous rod, and 

 the jaws are three in number. One kind has short stumpy jaws, 

 each with a poison bag at its base and a stiff stalk; these are 

 doubtless weapons of defence and enable the Urchin to give any 

 unwelcome visitor which may come too close a warm reception. 

 Such pedicellariae are called gemmiform (Fig. 161). Another 



