ECHINODERMA TA 



247 



walls of this the pharyngeal ossicles are formed ; these are 

 notched for the passage of the radial nerve and water- vascular 

 vessel. 



In some of the Synaptae the alimentary canal runs nearly 



#1^ 



^w^ 



Pig. 149. — Spicules of Holotlinroidea. 

 After Semper. 



d a and i. Anchor and anchor plate of 



Syiwpta mdivisa. Semper. 



c. Spicule of Chirodota rigida. 



d. Wheel spicule of Chirodota 



vitiensis. GrafFe. 



e. Spicule of Thyone chUenms. 



Semper. 

 /, g, h. Anchor and anchor plate of 

 Synapta godefroyn. Semper. 

 i. Spicule of Rhopalodima lageni- 

 formis. Gray. 



straight from the mouth to the anus, but as a rule it forms a 

 coil with three limbs. The mouth is situated in the centre of 



Fio. 150. — Diagi'am of 

 a transverse section 

 through the body of 

 a Holothurian. From 

 Leuckart. 



1. Dorsal surface, dorsal interradius. 



2. Ventral surface. 



3. Left dorsal radius. 



4. Bight dorsal radius. 



5. Right ventral radius. 



6. Left ventral radius. 



7. Radial nerve. 



8. So-called radial blood-vessel. 



9. Water-vascular vessel. 

 10. Radial muscles. 



-11" 



11', 11", 11'". The mesenteries of the 

 three limbs of the intestine. 



12', 12", 12"'- The three limbs of the 

 intestine. 



13. Respiratory trees, the left surrounded 



by a rete mirabile of blood-vessels. 



14. Two tubules of the generative organs 



lie to the right of the genital duct. 



15. Ventral blood-vessels. 



a peristomial area, and is in the Elasipoda directed ventrally. 

 The mesentery of the first descending limb of the alimentary 

 canal is situated in the interradius, in the middle dorsal line. 



