ECHINODERMATA 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 
Fria. 149.—Spicules of Holothuroidea. 
After Semper. 
q fea a and. Anchor and anchor plate of 
OWE Synapta indivisa. Semper. 
4) =I c. Spicule of Chirodota rigida. 
J Semper. 
d. Wheel spicule of Chirodota 
vitiensis. Griffe. 
e. Spicule of TZhyone chilensis. 
Semper. 
Jt, g, th. Anchor and anchor plate of 
Synapta godefroyii. Semper. 
i. Spicule of Rhopalodina 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 
Fic. 150.— Diagram of 
a transverse section 
through the body of 
a Holothurian. From 
Leuckart. 
5 
11" 12m 9 diac 
1. Dorsal surface, dorsal interradius. 11’, 11”, 11”. The mesenteries of the 
2. Ventral surface. three limbs of the intestine. 
3. Left dorsal radius. 12’, 12”, 12”. The three limbs of the 
4. Right dorsal radius. intestine. 
5. Right ventral radius. 13. Respiratory trees, the left surrounded 
6. Left ventral radius. by a rete mirabile of blood-vessels. 
7. Radial nerve. 14. Two tubules of the generative organs 
8. So-called radial blood-vessel. lie to the right of the genital duct. 
9. Water-vascular vessel. 15. Ventral blood-vessels. 
10. Radial muscles. 
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, 
