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 (Fig. 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 

 (p.d.a), posterior lateral (jhl.a), and post-oral {j^.v.a) processes. 

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

 oral ciliated rings there is an ectodermic thickening, recalling 

 the so-called apical plate of Annelid larvae. 



(2) The OpliiojihUeus, the larva of the Ophiuroidea, In 



Fig. 284.— a, Ophiopluteus of Ophiothrix fragilis. 

 hy, Hydrocoel ; Z.^j.c, left posterior coeloin ; oe/t, 

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

 stomach. B, metamorphosis of Ophioiiluteus 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-oxal, 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 tlie 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,^ wlio also discovered the other three types of 



^ "Die Echiiiodermenlarven der Plankton Expedition," Ergehn. Plankton Exp. 

 Bd. ii. J, 1898. 



- " tJber die Larvenzustande und die Metamorphose der Ophiuren und Seeigel," 

 Ahh. K. Akad. wiss. Berlin, 1846, and other papers in the same publication in sub- 

 sequent years. 



