207 
dehiscence of the ventral wall of the disk — as I have shown 
to be the case in Ophiopus arcticus x ), likewise wanting the bursæ — 
or perhaps the casting off of the disk is a normal process, the 
delivering of the genital products taking place in this way * 2 ). This 
cannot be ascertained on the preserved material. Jt should further 
be observed that the inner (first) tentacle is well developed. 
In the skeleton it may be observed that the “peristomial” 
plates are paired or wholly coalesced, though with an incision in 
the outer edge; both conditions may occur in the same specimen. 
Between the peristomial plates and the mouth-shield lies a small 
angular plate with a hole in it (PI. II. Fig. 5), conically elongated 
towards the ventral side. I am not sure of the morphological value 
which should be ascribed to this plate; as far as I am aware, a 
similar formation has not been described in other Ophiurids. 
The vertebræ of the arms are of a very peculiar and interesting 
structure. They are very elongate and siender, and their original 
paired condition is preserved in most of the length of the arm, 
only the ends of the ambulacrals being coalesced (PI. II. Figs. 6 — 7, 
Textfig. 2). The ambulacrals are parallel, the enclosed space being 
very narrow; the vertebræ differ thus conspicuously from those of 
Ophiohelus (Lyman “Challenger” Ophiuroidea. PI. XXVIII. Fig. 
10), in which the paired condition of the vertebræ is likewise 
preserved. The ligure 6, PI. II, shows the beginning coalescence 
of the vertebræ, fine outgrowths from the siender middle part of 
the ambulacrals meeting in the median line, forming a series of 
small holes. The coalescence gradually augments towards the base 
of the arms, but even the innermost, shorter and more robust 
vertebræ still show traces of the original paired condition. Also 
a ) Uber Ophiopus arcticus (Ljungm.) eine Ophiure mit rudimentaren 
Bursæ. Zeitschr. f. wiss. Zool. LVI. 1893 p. 566. 
2 ) Sluiter (On a probable periodical amputation of the disk-covering by 
some Ophiurids. Tijdschr. Nederl. Dierk. Vereen. 2. Ser. V. 1898. 
p. 306) makes the same suggestion for Ojjhiocnida echinata Ljn. It 
may perhaps be the case also in others of those, not few, species of 
Ophiurids which have the habit of throwing off their disk, as e. g. 
Amphiura fitiformis (O. F. Miill.). 
