6o6 



ECHINODERMATA 



out into short tag-like processes, in which we may distinguish 

 (following Mortensen's '^ notation) in the prae-oral loop an anterior 

 median process (Pig. 283, a.c.o.b), and a pair of prae-oral pro- 

 cesses (a.v.a). In the post-oral loop there is a median dorsal 

 process (p.c.o.b) and paired anterior dorsal (a.d.a), posterior dorsal 

 (ji.d.a), posterior lateral (p.l.a), and post-oral (p.v.a) processes. 

 At the apex of the prae-oral lobe between prae-oral and post- 

 ural ciliated rings there is an ectodermic thickening, recalling 

 the so-called apical plate of Annelid larvae. 



(2) The OpMopluteus, the larva of the Ophiuroidea. In 



post-oral. ami 

 postero. lateral arm 



Fia. 284. —A, Ophiopluteus of Ophiothrix fragilis. 



Hydrocoel ; l.p.c, left posterior coelom ; oes, 



oesophagus; r.p.c, right posterior coelom; st, 



stomach. B, metamorphosis of Ophiopluteus of 



Ophiura sp. (After Johannes Miiller.) 



this type the prae-oral lobe remains small, and the primitive 

 ■ciliated band is undivided. The processes into which it is 

 drawn out are very long, and are supported by calcareous rods. 

 Of these processes we may distinguish prae-oral, postero-dorsal, 

 postero- lateral, and post -oral. The postero- lateral are always 

 much longer than the rest, so that the larva when swimming 

 appears to the naked eye as a tiny V. In the case of 

 Ophiothrix fragilis (Fig. 284, A) the postero-lateral processes are 

 many times longer than the rest of the body. The Ophiopluteus 

 was the first Echinoderm larva to be recognised. It was discovered 

 by Johannes Miiller,^ who also discovered the other three types of 



' "Die Echinodermenlarven der Plankton Expedition," Ergebn. Plankton Exp. 

 Bd. ii. J, 1898. 



- " tiber die Larvenzustande und die Metamorpliose der Ophiuren und Seeigel," 

 Abh. K. Akad. loiss. Berlin, 1846, and other papers in the same publication in sub- 

 sequent years. 



