XVI ECHINODEEMATA 511 



string of cells ; this mass of cells hollows out to form a vesicle, the 

 madreporic vesicle, but the string of cells vanishes, and the right 

 coelom never exhibits a dilatation corresponding to that of the axial 

 sinus on the left side {m.v, Fig. 386). 



About the same time the longitudinal ciliated band becomes 

 thickened in four places, and bent outwards in the form of four 

 horizontal semicircular loops. Of these loops two are situated, one 

 on each side in the re-entrant angle of the ciliated band, between the 

 antero-lateral and postero-dorsal arms, and these are termed the dorsal 

 ciliated epaulettes ; and two, one on each side on the course of the 

 posterior cross-bar, which are termed the ventral ciliated epaulettes. 



These four ciliated epaulettes, distinguished as the anterior ciliated 

 epaulettes, become separated from the rest of the band, which heals 

 the breaches thus made in its continuity by special growth from the 

 broken ends (Fig. 386). The four ciliated epaulettes then grow till 

 they almost if not quite meet each other in the mid-dorsal and mid- 

 ventral lines ; they form the main locomotor organ of the later larva, 

 on which they bestow something of the aspect of a tiny medusa ; 

 they are much thicker, and carry much more powerful cilia than the 

 rest of the band. The aboral ends of all the skeletal rods undergo 

 resorption 



ECHINOPLUTEUS AND OPHIOPLUTEUS LARVAE COMPAEBD 



Leaving aside the ciliated epaulettes, which are by no means 

 universal in Echinoid larvae, and so far as yet known are confined to 

 the larvae of the genera Echinus and Strongylocentrotus, we may pass 

 in rapid review the chief differences between the Echinopluteus and 

 Ophiopluteus larvae at the height of their respective developments. 



The Ophiopluteus has only two calcareous centres for the larval 

 skeleton, one of which supplies the skeleton for all the larval arms of' 

 one side. The Echinopluteus has five calcareous centres, of which the 

 two primary supply the antero-lateral and post-oral arms on each 

 side, the two lateral supply the postero-dorsal arms, and the median 

 dorsal supplies the two prae-oral arms. 



The Ophiopluteus has no prae-oral arms, and possesses a pair of 

 postero-lateral arms, which are developed first and which are the 

 main organs of locomotion. The Echinopluteus has prae-oral arms, but 

 the post-oral arms are developed first, and when postero-lateral arms 

 are developed (they are absent in the larva of Hchinus esculentus, 

 but appear in the larvae of the Spatangoids) they appear very late, 

 and are the shortest of all the arms. 



It is obvious that the differences so far enumerated are of minor 

 importance, and would be such as one would expect to find separating 

 the larvae of two allied families. It is when we come to consider the 

 metamorphosis of the Echinopluteus larva that the radical differences 

 between it and the Ophiopluteus become apparent. 



