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pholis aculeata, apparently a near relative of it, has a simple body skeleton 

 tends to prove the same thing. 



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

 specialization in the Ophiocoma-lsirwse, in which they are so much reduced 

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

 of the skeleton. Otherwise 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 larvae of the 

 serratus-arcifer-grouTp; also Ophiopluteus monacanthus must evidently re- 

 present a remarkable specialization of this skeletal part. These median 

 processes are generally directed horizontally outwards from the transverse 

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

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

 produced along the body prominences formed by these rods. 



The remarkable preoral rod of Ophiopluteus coslatus, as yet quite a uni- 

 que feature in Ophiurid-larvse, may possibly have originated as a further 

 development from the median process; but nothing definitely can be 

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

 analogy between this preoral rod and the dorsal arch of Echinoid larvae 

 — but, of course, it is only a quite superficial analogy, by no means a 

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

 skeleton of Ophiurid- and Echinoid-larvae, the skeleton having certainly 

 been acquired separately in each of these two larval types. 



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

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

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

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

 lateral arms in forms hke Ophiopluteus pusillus may prove of some import- 

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

 rods supporting the anal area in Ophiopluteus fulcitus; but this is also a 

 feature as yet unique among Ophiurid-larvae, about the morphological 

 meaning of which it is not safe as yet to state anything definite. It recalls 

 the ventral transverse rods of Echinoid-larvae, with which there is, how- 

 ever, no homology, of course. 



The posterolateral rods show the greatest diversity of all skeletal parts 

 of the Ophiurid-larvae, and afford especially important diagnostic char- 

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

 thorns along the inner (adoral) side; more rarely thorns are found also 

 along the outer (aboral) side. A most remarkable speciaUzation is the un- 



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