140 



PHYLUM ECHTNODERMATA. 



8 



FIG. 95. Echinopluteus of a Spatangid (after J. 

 Miiller). 1 frontal area ; 2 preoral arm ; 3postoral 

 arm ; 4 anterior, 5 posterior transverse portion 

 of ciliated band ; 6 unpaired posterior ami ; 

 7 anal area ; 8 postero-lateral arm ; 9 oral area ; 

 10 postero-dorsal arm ; 11 antero-dorsal arm ; 

 12 antero-lateral arm. 



great development of the 

 postanal part of the body 

 (Fig. 94). The arms are 

 supported by calcareous 

 rods. 



The plutei of Ophiu- 

 roids and Echinoids are 

 distinguished as ophio- 

 plutei and echinoplutei 

 respectively. In ophio- 

 plutei the postero-lateral 

 arms are always the 

 largest and directed for- 

 wards (Fig. 94), and pre- 

 oral and antero-dorsal 

 arms are absent. In 

 echinoplutei (Fig. 95) the 

 postero-lateral arms when 

 present are directed back- 

 wards or outwards, preoral arms are present, and in Spatangid 

 larvae (Fig. 95) there is an unpaired posterior arm on to which 

 the ciliated band is not continued. Moreover in some genera 

 (Echinus, Strongylocentrotus, Sphaer echinus) there are in old 

 larvae four ciliated projec- 

 tions, called ciliated epau- 

 lettes (Fig. 96), at the base 

 of the postoral and postero- 

 dorsal arms, and sometimes 

 an additional pair at the 

 hind end of the body. 

 Ciliated lobes (auricular 

 appendages, auricles) are 

 cutaneous expansions be- 

 tween some of the arms in 

 certain echinoplutei, and 

 pedicellariae may appear 

 before the larval charac- 

 ters are lost. In ophio- 

 plutei the calcareous skele- FlG . 96 ._p lu t eus larva of Echinus KM with 



ton i=? in two Vmlvps parli four ciliated epaulettes TF (after Metschnikoff) 



-On IS m tWO naiVCS, eaCIl from the ventr al side. mouth ; A anus. 



u 



