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posterodorsal arms would appear to be absent. It is very tempting to sug- 

 gest that this larva may belong to Amphiura Chiajei. In that case the 

 absence of this pair of arms would seem to be characteristic of the Am- 

 phiura-larvde in general. However, the fact that among the numerous 

 Ophiurid-larvae examined by me from so many parts of the world, there 

 is no one lacking the posterodorsal arms, makes this suggestion very doubt- 

 ful, as it is, indeed, hard to assume that in all that multitude of forms 

 there should not be at least some larva of one or another of the numerous 

 Amphiura-species occurring in nearly every locality. But, anyhow, the 

 lacking of the posterodorsal arms, being quite a normal feature at least 

 in the larva of Amphiura filiformis, is a very noteworthy, probably regres- 

 sive, specialization. — In contradistinction to this stands the formation 

 of small extra arms in Ophiopluteus opulentus. It seems beyond doubt that 

 these arms are due to the branches from the postoral and anterolateral 

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

 as this is apparently the case also with the other arms, the growing skeletal 

 rod being the primary factor in the formation of the arms, it seems per- 

 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 to that of the other arms. 

 These additional arms of Ophiopluteus opulentus form an interesting analogy 

 with the formation of anterodorsal arms in the Spatangoid-larvae. 



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

 diversities within this larval type. Two main types are to be distinguished, 

 one having simple body rods, the other a compound body skeleton, a 

 ventral and a dorsal recurrent rod forming together with the body rod 

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

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

 body skeleton, the origin of which has been definitely ascertained, is that 

 of Ophiactis balli^); but it is clear that all the many difTerent forms having 

 a compound body skeleton cannot possibly belong to the family Ophiac- 

 tidae. It is evident, therefore, that this type of skeletal structure must have 

 developed independently along various fines, and accordingly the character 

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

 importance from a classificatory point of view. If I was right in referring 

 the Ophiopluteus gracilis, described in my Memoir on the Echinoderm 

 larvae of the German South Polar-Expedition (p. 89), to Ophiura gelida, 

 it becomes evident that the compound type of body-skeleton is at most 

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

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



') Th. Mortensen. On the development of some British Echinoderms; p. 11. 



