r20 



EMBRYOLOGY. 



the surface of the left water-tube w' ; the outline of a part of the right water- 

 tube (w) shows great increase in the volume of the tube. In an adult sea-ur- 

 chin pluteus [Fig. 55), the sea-urchin lias encroached so much on the anal ex- 



Fig. 55. treinity as to conceal 



the shape of the diges- 

 tive cavity. The spines 

 are so huge that we 

 are unable to trace the 

 position of the tentac- 

 ular system ; the anal 

 opening is very con- 

 spicuous. The vibratile 

 epaulettes (r") are re- 

 markably powerful. 

 The arms have attained 

 nearly the same length. 

 The vibratile chord has 

 been twisted in such a 

 manner as to assume 

 the appearance of bind- 

 ing an anal and an oral 

 plastron, of which e, e", 

 <■'" . and e' v are respec- 

 tively the arms; the 

 mode of formation of 

 the chord and of the 

 arms shows that all these anus in reality belong to one plastron (see Figs 

 47, 48), notwithstanding the great resemblance to the two distinct plastrons of 

 a Brachiolaria. Two very prominent black spots are seen in the arms e, e", 

 similar to those observed by Muller in his Pluteus quadrimaculatus ; a few 

 small spots are scattered over the other arms. The pluteus figured here in its 

 natural attitude does not undergo any further changes of form ; it now enters 

 a stage when the sea-urchin goes through its greatest transformations ; these 

 unfortunately cannot be followed, owing to the opacity of the embryo. 



The presence of rods in the plutean forms of Ophiurans and Echinoids of 

 course restricts considerably the play of the arms in assisting the motion of 

 the pluteus. The arms cannot be bent and twisted in the graceful manner so 



