I 82 



ECH1NODERMA TA. 



PART III. 



All parts of this creature, which is called a Pluteus, 

 are strengthened by a framework of calcareous rods 

 tipped with orange colour, all the rest being transparent 

 and colourless. It swims freely, back foremost, by means 

 of its cilia. 



While in this active state a circular disk (c, fig. 142), 

 covering the stomach (6, fig. 142), appears within it, 

 which gradually expands, and sends through the skin of 



the Pluteus spines, 

 pedicellarise, andtu- 

 bercules, ultimately 

 developed into hol- 

 low feet. Then the 

 feet are pushed out 

 and drawn in, the 

 pedicellarise (D, fig. 

 143) snap their pin- 

 cers ; and while the 

 half-formedEchinus 

 is making these mo- 

 tions within the 

 Pluteus, the mouth 

 and gullet of the Pluteus itself are in constant activity ; 

 and, while it swims about, the unformed Echinus within 

 it gets a globular shape, the shell is formed, and when 

 the Echinus is complete, the rest of the Pluteus is 

 thrown off, and the young animal rolls away. 



The free swimming larval zooids of the Echinodermata 

 are generally hyaline, and some are phosphorescent. 

 The Pluteus is also the larval zooid of the ophiurid star- 

 fishes ; they may be seen in great numbers on the surface 

 of the sea in August and September. The young star- 

 fish is formed in them by a process analogous to that 

 described. The motions of the Echinidse are reflex; 

 nothing indicates volition. 



The fossil Echinidse first appeared in the lower Lud- 



Fig. 143. Larvae of Echinus in various stages of 

 development within the Pluteus, which is not 

 represented : B, disk with the first indication 

 of the cirrhi ; c, disk with the origin of the 

 spines between the cirrhi ; D, more advanced 

 disk with the cirrhi, g, and spines, x, projecting 

 from the surface. 



