211 



are reorganized, acquiring the shape of a homogeneous cellular mass in 

 which, at a later stage, lumens appear and the epithelial structure is restored. 

 At this time, formation of the oral disk proceeds on the ventral side concomi- 

 tant with ectodermal thickening. The oral opening lies in the center of this 

 disk and is covered by the peristomial membrane. The peristomial membrane 

 delimits the peristomial cavity in which the dental apparatus develops after 

 the larva has settled. Brittle stars lack an anal opening (McBride, 1907; 

 Narasimhamurti, 1933; Olsen, 1942). 



Respiration : The respiratory functions are passed on from the larval 

 epithelium to the definitive epithelium, including the epithelium covering the 

 ambulacral podia. The transport fiinctions are transferred to the hemal and 

 perihemal systems and the visceral coelom. Spheroid bodies with phagocytes 

 and, possibly the axial organ, take part in excretion processes. Considerable 

 modifications of the coeloms lead to the formation of an ambulacral system, 

 hemal and perihemal systems, and perivisceral coelom of the definitive indi- 

 vidual. 



Locomotion : During metamorphosis, the locomotor fiinctions are passed 

 on from the ciliated band to the first terminal and first ambulacral podia; in 

 lecithotrophic larvae of Ophiura brevispina and Ophiolepis elegans the two 

 locomotor systems fiinction simultaneously (Strathmann, 1978). Later, the 

 ambulacral system loses its leading role in locomotion in brittle stars and the 

 mobile arms of the definitive brittle stars act as the principal locomotor 

 organs. 



Nervous system : The epineural radial and ring canals form under the 

 lateral processes of the hydrocoel and ring canal of the ambulacral system. 

 The ectoneural system passes in the canals of the epineural system. The 

 hyponeural system develops in the walls of the perihemal canals (McBride, 

 1907; Olsen, 1942). 



Reproductive system : The primary gonads appear in the walls of the left 

 somatocoel adjacent to the stone canal and begin to form the gonadial strand 

 after the juvenile individual settles (Figure 165). 



Settling : Development of the juvenile individual proceeds during the 

 planktonic larval period. Completely formed individuals settle on the sub- 

 strate. No attachment organs are present in brittle stars. There is no reliable 

 information on ability to search the substrate before settling (Strathmann, 

 1978). The juvenile brittle stars, on settling, attach themselves to the substrate 

 by primary ambulacral podia. Usually, by this time, nearly all the larval arms 

 are frilly resorbed. However, McBride (1907) noted that in Ophiothrix fragilis 

 the posterolateral arms are shed after the juveniles settle on the bottom. Thus, 



