Fig. 19.—Sequence of Ophioplutei showing successive stages in the reduction und loss of the larva. a, first stage—-e.g., Ophiura, albida; b, second stage—e.g., Amphiura filiformis; c, third stage— e.g., Ophiura affinis; d, fourth stage—e.g., Ophiopluteus claparédei; e, fifth stage, Amphipholis squamata; f. final stage, lurva absent and development direct—e.g., Kirk's ophiuroid. The number above the larval arms indicate the order in which these organs disappear from the larva.

Next we may select the larva believed to belong to Ophiura affinis (Fig. 19c), in which the postoral arms have disappeared altogether, and the antcrolateral pair also. Thus the last pair to disappear will be the posterolateral.

In the larva known as Ophiopluteus clajtaredei (the parent species being unknown) we have the stage in which the posterolaterals have indeed disappeared. That the posterolaterals should be the last to go is of particular interest, because in the metamorphosis of Ophiothrix fragilis it is this pair which is the last to be lost. This peculiar armless larva was taken by Claparede (1863) swimming on the surface of the sea off the coast of Normandy. In his description

of the animal he confuses the hydrocoel with a developing young ophiuroid, owing to its five lobes, but his accurate rendering of the organ in the figure makes clear its true nature. He records that there is a mouth opening, but apparently the anus has disappeared for he does not figure or mention it. At the aboral end are at either side two spicules, clearly vestiges of the skeletal rods of the arms. At the aboral apex, and on each of the two pairs of projecting “shoulders” are tufts of cilia. There can be no doubt that these shoulders represent the two main regions whence the arms arise in the fully developed species. But the most significant fact recorded in his account is the fact that the larva was so opaque (“undurchsichtig”) that the internal organs were somewhat obscured. Now this opacity undoubtedly indicates the presence of yolk in the tissues, and here we have the first indication of the point in the series at which increasing yolk began to be of importance. The reduction in the alimentary canal is complementary to the presence of yolk (Fig. 19d).

A further step in this sequence brings us to such forms as Amphipholis squamata where the reduction has proceeded so far as to obliterate all traces of arm roots and cilia, leaving only a vestigial pair of skeletal meshes, recognisable as vestiges of the arm rods by their position and inclination, but having lost the slender spicular form. The mouth opening has now disappeared, the alimentary canal being thus vestigial. The yolk mass has increased so as to make the larva quite opaque until artificially cleared (Fig. 19e).

The endpoint in the series is represented by such forms as Kirk's ophiuroid, in which the larval stage has disappeared entirely from development (Fig. 19f).

For this process of shifting backwards of the time at which radial form is assumed I suggest the term “Recession of Metamorphosis”.

A further indication that this has actually occurred is provided by the hydrocoel. As is well known, the time at which metamorphosis first commences is indicated by the behaviour of the hydrocoel. For this organ moves from its position on the left side of the gut and begins to encircle the oesophagus, its five lobes becoming the five radial canals of the adult. As recession of metamorphosis proceeds, the encirclement takes place relatively earlier in the life cycle, till in the directly developing form it appears right from the beginning as a canal encircling the future oesophagus (Fell, 1941).

This process of earlier and earlier metamorphosis is in reality a species of neoteny—the larva becoming “adult,” as it were, at successively earlier and consequently undeveloped stages. A general diagram (Fig. 20) illustrates graphically the effect of recession of metamorphosis in ophiuroids.

It might be argued that the sequence of larval forms described above represents not a regression but a progression. This view would regard the fully developed Ophiopluteus as being a later evolutionary product derived from the simpler forms of larva, such as is found in