18 ExMBRYOLOGY OF THE STARHSH. 



anterior and posterior one. In the embryo of our Starfish, -which told 

 so plainly, in its early stages, of the plan upon which it is built, that 

 plan is now lost sight of in the extraordinary bilateral development of 

 some of the parts. But, until Spatangoids and fiat-soled Holothurians are 

 proved to be truly bilateral animals, and not genuine Radiates, with sub- 

 ordinate bilateral features, these seeming bilateral Echinoderm larvas must 

 be considered as truly radiate, with bilateral features engrafted upon 

 them. 



Devehpmeiit of the Plastrons. — The cylindrical shape, characterizing the 

 earlier stages of the larva, disappears soon after the appearance of the 

 first trace of the appendages which give to these larvae such a peculiar 

 appearance, and they now assume the features of the adult. The depres- 

 sion (PI. ir. Fi(/s. 25, 27, m), in which the mouth is placed, becomes more 

 marked ; we have a greater separation of the oral (v) and anal {v) swell- 

 ings of the vibratile chord, little by little changed into two independent 

 breastplates, the edges bound with chords of powerfid vibratile cilia, be- 

 coming the locomotive organs of the larva3 (PI. II. Figs. 20, 22, 24, 26, 

 28). These plastrons, at first mere crescent-shaped shields (PI. II. Figs. 

 20, 22, 24), extend gradually towards either extremity, become elliptical, 

 and then somewhat triangular. The outline of the anal shield becomes 

 sinuous, slight indentations point out the position of the future arms (PI. 

 II. Fig. 26, c e, Fig. 28, e e, e" c") ; the rows of cilia creep gradually 

 round the edge of this anal shield, turn towards the mouth again, and 

 extend, on the dorsal side, along the whole length of the larva (PI. II. 

 Fig. 25) ; this chord of cilia makes a complete circuit, while the cilia, 

 extending along the edge of the oral plastron, do not meet. 



The formation of these plastrons is attended with great changes in the 

 general outline of the larva ; the anal extremity becomes pointed, trian- 

 gular, with rounded edges; the body, on each side of the oral opening, 

 bulges out beyond the general outline, and the oral plastron is more and 

 more pointed, as it separates from the rest of the larva. This change of 

 .shape can perhaps be better appreciated when seen in profile, and by 

 comparing the drawings of larvae three days and six days old ; compare 

 PI. 11. Fig. 19 with PI. II. Figs. 25 and 27 seen from opposite sides. The 

 great elongation of the oral extremity and the marked separation made 

 by the ()|)(Miing of the mouth between the anal and oral plastrons cannot 

 la 11 to be noticed. 



