226 



poslero<l()is;il ;iiins would iippcar lo he ;il)s('iil. II is vt'ry leinpUng to sug- 

 gest IIkiI this l;irv:i iikiv holoiig lo Amphiuni C.hinjvi. In that case the 

 al)sc'iici' ol I his pair ol' arms would sccni lo be characlerislic of Ihe .4m- 

 pliinni-hiiyiv in general. I lowever. Ihe I'arl Ihal among Ihe numerous 

 ()|)hiuri(l-larvcV examined h\ me Iroiii so many j)arls of Ihe world, liiere 

 is MO one lacking Ihe poslerodorsal arms, makes I his suggeslion very donbl- 

 ful, as il is, indi'cd, hard to assiiiuc Ihal in all that mnllilnde nt loinis 

 there should nol he at k-asl some lar\a of oni' or anothei' of the numerous 

 .l/;i/>/)///r«-speeies oeeurring in nearly every locality. But, anyhow, the 

 lacking of the posterodoisal arms, being (piite a normal feature at least 

 in the larva of Amphiuia /ilifoimis, is a very nolewcirthy, probably regres- 

 sive, specialization. In conlradistinction lo I his stands the formation 

 of small e.xira arms in Ophioplulcus opulcnliis. 11 seems beyond dnui)l Ihal 

 these aims are due lo the branches from Ihe postoral and anterolateral 

 rods pushing out the vibratile band so as to form a beginning arm; but 

 as this is ap|)arently the case also with the other arms, the growing skeletal 

 rod being the primary factor in the formation of the arms, it seems |)er- 

 fectly justified to regard these small extra arms as having the value of 

 true arms, and one might very well fancy that other larval forms may 

 exist in which these arms reach a size similar lo that of the other arms. 

 These additional arms of ()])hiophilcus ojndviilus form an interesting analogy 

 with Ihe formation of anterodorsal arms in the Spatangoid-larva'. 



It is, however, the skeletal structure that shows the more interesting 

 diversities within this larval type. Two nuun types are lo be distinguished, 

 one having simple body rods, Ihe oilier a compound body skeleton, a 

 ventral and a dorsal iccurrenl rod forming together with Ihe body rod 

 two meshes in each side of the body. It would seem fairly certain Ihal 

 the simple type is the more primitive. The only larva with a compound 

 hotly skeleton, the origin of which has been definitel\' ascertained, is that 

 of Ophiactis balli^): but it is clear that all the many dilTerent forms having 

 a compound body skeleton cannol possibly belong lo the family ()|)hiac- 

 tida^. It is evident, Ihei-efore, Ihal this type of skeletal structure must have 

 develoj)ed independently along various lines, and accordingly the character 

 of the body skeleton, whether simple or compound, cannot be of primary 

 im|)()rtance from a classilicalory point of view. If I was right in referring 

 the Ophioplulcus (jracilis, described in my Memoir on the Echinoderm 

 larvae of the (ierman South Polar-I^xpedition (p. 89), to Ophiuni (jdida, 

 it becomes evident that the compound l\pe of body-skeleton is at nu)sl 

 a genus-character, not a family-character. The fact that the larva of 

 Ophiaclis balli has a compound body skeleton, while the larva of Ophio- 



') Th. Mortenscn. On the development of some British Ei-hiiioderms; p. 11. 



