ASTEKOIDEA. 247 



surface, united along the middle line by their inner ex- 

 tremities. They are so placed as to form a kind of elongated 

 pent-house, and immediately beneath the line where the 

 ossicles of one side are articulated with those of the other 

 side is placed the ambulacral vessel (5). Superficial to this, 

 again, is a nerve-cord ; so that the whole chain of ambulacral 

 ossicles is placed in the midst of the soft parts of the animal, 

 and is thus clearly an internal skeleton. At their outer ex- 

 tremities the ambulacral ossicles are articulated by the inter- 

 vention of the " adambulacral plates" (fig. 140, &), with 

 plates belonging to the external or integumentary skeleton, 

 to be immediately described. As before said, the shape of 

 the ambulacral ossicles is such that a pore is formed by the 



Fig. 141. Section of the ray of Uraster rubens. a, a, Ambulacral ossicles ; 5, Position of 

 the ambulacral vessel ; c, c, Plates of the external skeleton ; n, Nerve-cord. The dotted lines 

 show the tube-feet proceeding from the ambulacral vessel. 



apposition of each pair ; and by these apertures each tube- 

 foot communicates with a vesicle placed internal to the 

 chain of ossicles. It will be seen, however, that the tube- 

 feet (indicated by the dotted lines in the figure) do not pass 

 through these apertures, or through any other pores of the 

 skeleton, on their way to the surface. The "poriferous 

 zones " of the Sea-urchins are part of the external skeleton, 

 and are not represented in the Star-fishes. On the other 

 hand, the integumentary skeleton in the Star-fishes is absent 

 along the ambulacral areas, or along the areas occupied by 

 the ambulacral grooves. 



Leaving the ambulacral ossicles or internal skeleton of the 

 Asteroidea, we come now to the integumentary skeleton. 

 This consists of a vast number of small calcareous pieces, 



