246 



ASTEROIDEA AND OPHIUROIDEA. 



number with the arms, and termed the " ambulacral grooves." 

 Each ambulacral groove is continued along the lower surface 

 of one of the arms, tapering gradually towards the extremity 

 of the latter. The floor of each groove is constituted by a 

 double row of minute calcareous pieces the "ambulacral 

 ossicles " which are movably articulated to one another at 

 their inner ends. At the bottom of each groove is lodged 

 one of the radiating canals of the water-vascular system or 

 ambulacral system, from which are given off the rows of 

 suctorial feet, or " tube-feet." It follows from this that the 

 radiating vessels of the ambulacral system are outside the 

 chain of ambulacral ossicles, so that these latter are to be 



regarded as an internal skel- 

 eton, and they do not corre- 

 spond with any part of the 

 skeleton of Echinoids 1 at 

 least they do not correspond 

 with the perforated ambu- 

 lacral plates of the Sea- 

 urchins. The ambulacral os- 

 sicles, however, of the' Star- 

 fishes are of such a form that 

 by their apposition an aper- 

 ture or pore is formed between 

 each pair. By means of these 

 pores (fig. 140, a) the tube- 

 feet communicate with a series 

 of little bladders, or "am- 

 pullae," placed above the chain of ossicles. These perforations, 

 however, do not correspond 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. 141). In such a section the am- 

 bulacral ossicles (, a) are seen in the centre of the lower 



1 The structures in the Echinus, which are truly homologous with the 

 ambulacral ossicles of the Asteroidea and Opkiuroidea, are the so-called 

 " auriculae." 



Fig. 140. Diagram of a Star-fish (Goniaster), 

 showing the under surface, with the mouth 

 and ambulacral grooves, a, Ambulacral os- 

 sicles, with the ambulacral pores between 

 them ; 6, Adambulacral plates, bounding 

 the ambulacral grooves ; m, Marginal plates 

 (wanting in many species) ; o, Oral plates, 

 placed at the angles of the mouth. 



