ASTEROIDEA. 113 



section of the arm of a Star-fish from which the soft parts have 

 been removed (fig. 67). In such a section the ambulacral 

 ossicles (a, a] are seen in the centre of the lower surface, 

 united along the middle line by their inner extremities. 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 (fr). 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 extremities the ambulacral 

 ossicles are articulated by the intervention of the " adambulac- 

 ral plates " (fig. 66, ), 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 



Fig. 67. Section of the ray of Uraster nibens. a, a Ambulacral ossicles ; b 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 



pore is formed by the 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, or " os- 

 sicles," united together so as to form a species of chain-armour. 

 H 



