MONOGRAPH OF JAPANESE OPHIUROIDEA. 371 



separation of the same is an archetypal but not embryonal con- 

 dition. The interpretation put forth in this and the last paragraph 

 is well in accord with the biogenetic law as formulated by von 

 Baer but not as formulated by F. Müller. 



In Ophiolepfoplax and Ophiohyalus, the dorsal arm plates are 

 present but very rudimentary, thin and hyaline. A similar con- 

 dition may be observed also in the distal arm joints of Ophiomyxa, 

 as well as Ophiodera anisacmitha, though the plates are in these 

 forms divided into several secondary plates in the proximal arm 

 joints. I imagine that, these dorsal arm plates may represent an 

 archetypal condition. 



As to the divided vertebrae, those of tlie Ophiomyxince appear 

 to me to be more archetypal than tliose of the Ophîacanthidœ, 

 the former reminding us of those of such a Palaeozoic form as 

 Ophiur'ina. 



Upon the basis of these considerations, I imagine that the 

 Ophiomyxince are a step more archetypal than the Ophiacanthidce. 

 Those genera of the Ophiomyxince, in which the second oral 

 tentacle pores open outside the oral slits, have usually very slender 

 adorai shields and very small oral shields, reminding us of the 

 Palaeozoic Myophiurolda, which are characterised by the second 

 oral tentacle pores opening outside the oral slits, by the adorai 

 shields not being well differentiated from, but almost similar in 

 shape to, the ordinary lateral arm plates, and by the absence of 

 the oral shields. That the second oral tentacle pores open outside 

 the oral slits is a character, which appears to be at once archetypal 

 and embryonal. 



The majority of those genera of the Ophiacanthidce, in which 

 the arms are only horizontally flexible, have more or less well 

 developed dorsal and ventral arm plates, which separate the lateral 



