![Thumbnail: [Volume 75, 1945-46 page 451]](/journals/rsnz/images/rsnz_75/thumbnails/rsnz_75_04_0569_0451_ac_02.gif)
The Metamorphosis.
The description of the development of the embryo has been brought up to the stage of the fully-formed larva. The transformation of this simplified, but nevertheless easily recognisable pluteus with bilateral symmetry into the radially symmetrical ophiuroid can now be described.
Fig. 14.—Larva viewed from the left side, at the commencement of metamorphosis.
Emb.At., embryonic attachment; Lft.Skel.Pl., left skeletal plate of the larva; Stom., stomach; Hydr., hydrocoel; Oes., oesophageal sac; Mesench., mesenchyme; Skel.Rud.Ad., triradiate skeletal rudiment of the adult; Rt.Skel.Pl., right skeletal plate of the larva.
As usual in the ophiuroids, metamorphosis seems to be initiated by a change in the position and shape of the hydrocoel. The organ begins to curve round the oesophagus (Fig. 14) so that the five lobes originally placed in linear series on the left side now begin to take up a position such that each lobe begins to point outwards, the lobes being at equally spaced intervals about the oesophageal sac. Not all stages of this process have been obtained, but there is no reason to believe that there are any important differences from what has already been described for Ophiothrix fragilis by MacBride (1907) and Ophiocomina nigra by Narasimhamurti (1933). No doubt the change in the shape and relations of the hydrocoel is brought about by the liberation of a growth substance, as first recorded in echinoplutei by Huxley (1928). The ring canal becomes established about the oesophagus when the two extremities of the hydrocoel meet
![Thumbnail: [Volume 75, 1945-46 page 452]](/journals/rsnz/images/rsnz_75/thumbnails/rsnz_75_04_0570_0452_ac_02.gif)
and fuse on the right side of the oesophagus, whilst the five lobes already present form the five radial ambulacral canals. Nothing was found comparable with the peculiar process of rotation through 360° of the hydrocoel of Ophiura brevispina, as described by Grave (1900). While the hydrocoel is encircling the oesophagus, there occurs a flattening of the embryo, shortening the antero-posteral axis and causing the embryo to become more spherical in form. The anterior surface of the embryo becomes at the same time rather flattened.
It thus happens that the anterior (or apparent anterior) hemisphere of the larva becomes directly transformed into the ventral half of the young ophiuroid, and the posterior hemisphere becomes dorsal. The correspondence of the regions, however, is not an exact one, for the actual posterior pole of the larva is found after metamorphosis to have become dorso-lateral, being midway between the central point of the dorsal surface of the star and the periphery of one interradius. The only landmark of value which remains unaltered during the metamorphosis is the embryonic attachment—which, it will be recalled, is united to the posterior pole, thereby providing a useful index to the position of that region. This organ therefore comes to be dorsal and interradial in the young attached ophiuroid. The interradius to which the embryonic attachment is joined is always either of the two interradii which are furthest from the sector subsequently occupied by the madreporic canal (sec Fig. 20).
Fig. 15.—Vertical section through a newly-metamorphosed star, still attached to the parent.
Pod, podium; Amb.C., ambulacral canal; Mesch., mesenchyme; Coel.Sp., intercellular schizocoelous splits forming the perivisceral coelom; Stom., stomach; Emb.At., embryonic attachment; Ect., ectoderm; Amb.R., ambulacral ring; Oes.Rud., oesophageal rudiment; Mo.Rud., mouth rudiment.