464 



BIPINNARIA. 



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 ; mx. amoe- 

 boid cells. 



to 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 larvae of Holo- 

 thurians proper use their embryonic 

 tube feet to creep about. 



Asteroidea. The commonest 

 and most thoroughly investigated 

 form of Asteroid larva is a free 

 swimming form known as Bipin- 

 naria. 



This form in passing from the 

 spherical to the bilateral condition 

 passes through at first almost 

 identical changes to the Auricu- 

 larian larva. The cilia become 

 at an early period confined to an 

 oral and anal ridge. 



The anal ridge gradually ex- 

 tends dorsalwards, and finally 

 forms a complete longitudinal 



post-oral ring (fig. 259 A) ; the 

 oral ridge also extends dorsalwards, and forms a closed praeoral ring 

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

 figure. 



The presence of two rings instead of one distinguishes the Bipin- 

 naria from the Auricularia. The two larvse 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 larvse provided with them as Brachiolaria (fig. 259 D). 



As a rule the following arms can be distinguished (fig. 259 C and D), 

 on the hinder ring (Agassiz' nomenclature) a median anal pair, a dorsal 

 anal pair, and a ventral anal pair, a dorsal oral pair, and an unpaired 

 anterior dorsal arm ; on the prreoral ring a ventral oral pair, and sometimes 

 (Muller) an unpaired anterior ventral arm. 



The three brachiolar arms arise as processes from the base of the un- 



