1 66 MANUAL OF ZOOLOGY. 



bulacral ossicles, however, of the Star-fishes are of such a form 

 that by their apposition an aperture or pore is formed between 

 each pair. By means of these pores (fig. 59, a) the tube-feet 

 communicate with a series of little bladders placed above the 

 chain of ossicles. These perforations, however, do not corre- 

 spond with the perforated plates of the Echinoid test, and the 

 tube-feet of the Star-fishes pass through no " poriferous " plates 

 on their way to the exterior. 



This may be rendered more intelligible by examining a 

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

 been removed (fig. 60). 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 



Fig. 60. Section of the ray of Uraster ritbens. 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. 



immediately beneath the line where the ossicles of one side 

 are articulated with those of the other side is placed the 

 ambulacral vessel (b). 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. 59, b), 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 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 



