Fig. 258. A late stage in the development of Synapta. (After Metschnikoff.)
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. amœboid cells.
Asteroidea. The commonest 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 præ-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 larvæ 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 known as brachiolar arms, and the larvæ provided with them as Brachiolaria ([fig. 259] D).