A8TERIAS 



6. Transverse section of the decalcified arm of a small specimen. Body- wall ; spines ; dermal 

 branchiae ; ambulacral ossicles ; neural epithelium ; radial water-vascular canal ; ampullae ; tube-feet ; 

 body-cavity; hepatic caeca and their mesenteries; in sections near base of arms, generative organs. 



Examine sections of an undecalcified arm to show ambulacral ossicles. 



7. Region of ambulacral groove under high power. Columnar epithelial cells; internal pro- 

 longations of same ; sections of nerve-fibres and nerve-cells between the latter ; septum containing 

 the so-called radial blood-vessel ; radial water-vascular trunk ; ambulacral ossicles ; ampullae ; tube- 

 feet. 



8. Transverse section of stone-canal and heart. 



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