ECHINODERMATA. 



557 



be five plates surrounding the 

 mouth, which are developed before 

 any other part of the skeleton, and 

 are regarded by P. H. Carpenter 

 (No. 548) as equivalent to the five 

 oral plates of the Crinoidea. The 

 larval condition with ciliated bands 

 is often spoken of as the pupa stage, 

 and during it the larvas of Holo- 

 thurians proper use their embryonic 

 tube feet to creep about. 



Asteroidea. The com- 

 monest and most thoroughly 

 investigated form of Asteroid 

 larva is a free swimming form 

 known as Bipinnaria. 



This form in passing from 

 the spherical to the bilateral 

 condition passes through at 

 first almost identical changes 

 to the Auricularian larva. 

 The cilia become at an early 

 period confined to an oral 

 and anal ridge. 



The anal ridge gradually extends dorsalwards, and finally 

 forms a complete longitudinal post-oral ring (fig. 259 A) ; the 

 oral ridge also extends dorsalwards, and forms a closed prae-oral 

 ring (fig. 259 A), the space within which is left unshaded in my 

 figure. 



The presence of two rings instead of one distinguishes the 

 Bipinnaria from the Auricularia. The two larvae are shewn side 

 by side in fig. 254, and it is obvious that the two bands of the 

 Bipinnaria are (as pointed out by Gegenbaur) equivalent to the 

 single band of the Auricularia divided into two. Ontologically, 

 however, the two bands of Bipinnaria do not appear to arise 

 from the division of a single band. 



As the Bipinnaria grows older, a series of arms grows out 

 along lines of the two ciliated bands (fig. 259 C), and, in many 

 cases, three special arms are formed, not connected with the 

 ciliated bands, and covered with warts. These latter arms are 



FlG. 258. A LATE STAGE IN THE DE- 

 VELOPMENT OF SYNAPTA. (After Metschni- 

 koff.) 



The figure shews the vestibular cavity 

 with retracted tentacles ; the ciliated bands ; 

 the water-vascular system, etc. 



p. dorsal pore of water-vascular system ; 

 pv. walls of perivisceral cavity ; ms. amoe- 

 boid cells. 



