SPINES 



455 



paratively short, blunt tubercles, covered with thick skin. In 

 the Echinasteridae and Asterinidae they are short and blunt, 

 but they are very numerous and thick set. In the Solasteridae 

 they are long, and arranged in bundles diverging from a 

 common base. Such bundles may be termed sheaves, and 

 starting from an arrangement like this, two distinct lines of 

 modification may be traced. Thus (1) the members of a sheaf 

 become connected by a web of skin, so that the sheaf becomes an 

 umbrella, and successive umbrellas may adhere, so that a supra- 



FlG. 194. Views of portions of the aboral surface of different genera of Asteroidea 

 in order to show the main varieties of skeleton. A, Solaster, showing spines arranged 

 in sheaves ; B, Pteraster, showing webs forming supra-dorsal membrane supported 

 by diverging spines ; C, Astropecten, showing paxillae ; D, Nardoa, showing uniform 

 plating of granules. x 8. (After Sladen.) 



dorsal tent is formed (a structure characteristic of the Pter- 

 asteridae), or (2) the members of a sheaf may become arranged 

 in a circle round a central vertical axis so that a structure like a 

 capstan is produced, which is called a " paxilla " (characteristic of 

 Astropectinidae, Porcellanasteridae, and Archasteridae). The 

 axis, 1 as shown by its development, represents the plate which 

 bore the bundle of spines. Again, the skeleton may consist of 

 plates with a close covering of granules (Pentagonasteridae, etc.). 

 Lastly, in Porania spines are absent, the plates being deeply 

 embedded in a thick leathery skin. 



1 Ludwig, "Die Echinodermen des Golfes von Neapel," pp. 68, 69. 



