1869.] BILLINGS — ON THE STRUCTURE OP CRINOIDEA, ETC. 427 



2. 



Fig. 1. The upper part of Caryocrinus ornatus, the test being removed 

 in order to show the internal structure of the fourteen hydrospires 

 that surround the summit. The parallel lines represent the flat 

 tubes. The other figures exhibit the modifications which the 

 hydrospires undergo in passiilg through :— 2. Codmter. 3. Pen- 

 tremites with broad ambulacra. 4. Fentremites with single tubes. 

 5. Paloezmc Crinoids with a convoluted plate attached to the 

 centre of radiation. 6. Sand canal or madreporic tube of a 

 starfish inclosing a doubly convoluted plate 7. Ambulacral 

 canals of a starfish with the doubly convoluted plate ot the sand 

 canal attached to the oesophageal ring. The following letters 

 have the same reference in all the figures in which they occur: a, 

 an arm or ambulacrum ; ?m v, moHth and vent combined in a single 

 aperture -.^w^s, mouth, vent and spiracle ; g, ambulacral groove; p, 

 ovarian pore ; s, spiracle ; cjp, convoluted plate ; r, oesophageal ring. 



