112 



ANNULOIDA. 



entrance to the water-vascular system. Ordinarily there is a 

 single madreporiform tubercle, but in some genera there are 

 two, three, or more tubercles ; and there seems in some cases 

 to be a correspondence between the number of the arms and 

 the number of madreporic plates. 



Placed in the centre of the lower surface is the mouth, at 

 the angles of which are the so-called "oral plates" (fig. 66). 

 Radiating from the mouth are a series of furrows, varying in 

 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 am- 

 bulacral system, from which 

 are given off the rows of suc- 

 torial feet, or " tube feet." 

 It follows from this that the 

 radiating vessels of the am- 

 bulacral system are outside 

 the chain of ambulacral os- 

 sicles, so that these latter 

 are to be regarded as an 

 internal skeleton, and they 

 do not correspond with any 

 part of the skeleton of Echi- 

 noids * at least they do 

 not correspond with the per- 

 forated ambulacral plates of 

 the Sea-urchins. The am- 

 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. 66, 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 



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

 ambulacral ossicles of the Asteroidea and Ophiuroidea^ are the so-called 

 "auricube." 



Fig. 66. 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. b, Adambulacral plates, bounding 

 the ambulacral grooves ; m, Marginal plates 

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

 placed at the angles of the mouth. 



