227 



pholis aculealn, apparent 1> n near relative of it. has a simple body skeleton 

 lends U) piove the same tiling. 



The end rods and the transverse rods reach an extraordinary degree of 

 specialization in llie Ophiocoma-larvce, in which they are so much reduced 

 in size as to form together merely a sort of link between the two halves 

 of the skeleton. Oilierwise the end rods show only a small amount of 

 diversity, less so than the transverse rods which often afford important 

 specific characters. More interest, however, is attached to the median pro- 

 cesses from the transverse rods, present in a number of forms; these rods 

 may show a high degree of development, especially in the l;ii\;i' of llie 

 si'iidlus-dirifei-group; also Ophiopluteus munacanlhus must evidently re- 

 present a remarkable specialization of this skeletal part. These median 

 l)rocesses are generally directed horizontally outwards from the transverse 

 rods and thus considerably enlarge the bod} volume, which may serve to 

 increase the floating power of the larva. In no case the vibralile band is 

 produced along the body prominences formed by these rods. 



The lemarkable preoral rod of Ophiopluteus cosUitus, as yet quite a uni- 

 que feature in ()])hiuri(l-larva', nmy possibly have origiiuded as a further 

 develojiment fi-om the median process; but nothing (ielinitely can be 

 stated about this at our present state of knowledge. There is a certain 

 analogy between this |)reoral rod and the dorsal arch of Echinoid larvae 

 — but, of course, il is only a (|uite superficial analogy, by no means a 

 homology, as, upon the whole, there is no direct homology between the 

 skeleton of ()])hiuri(l- and Echinoid-larvie, the skeleton having cOrlainlx 

 been acquired separately in each of these two larval types. 



The rods of the inner pairs of arms (postoral, anterolateral and postero- 

 ilorsal) show but little diversity. The various degree of development of 

 their thorns is a featuie of interest here; it is not improbable that the strong 

 development of ilioins along the aboral side in the outer part of the antero- 

 lateral arms in forms like Ophioplulcus pusillus may prove of some import- 

 ance. A more conspicuous feature is, however, the development of sjiecial 

 rods supporting the anal area in Ophioplulcus {ulrilus; bul this is also a 

 feature as yel unicjiu' among Ophiurid-larva', about the nu)i|)hological 

 meaning of which il is not safi' as yet to slate anything delinitc. It iccalls 

 the NiMitial transverse rods of Mchinoid-larva*, with which there is. how- 

 evei', HO iiomology, of course. 



The |)osterolateraI rods show the greatest diversity of all skeletal parts 

 of the Ophiurid-larv.e, and afford especially important diagnostic char- 

 acters. The mori' usual type is a simple rod provided with strong, curved 

 thorns along llie inner (adoral) side; more raiely thorns are found also 

 along the outer (aboral) side. .\ most remarkable specialization is the un- 



