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7. The Vitellaria or “Yolk-Larva” Series.
The bioseries we have just considered covers most of the ophiuroids. There remains, however, a peculiar divergent series which can be correlated with no larval series at all, but which nevertheless involves free-swimming larval forms with unmistakable characters in common. The amazing fact which emerges from a consideration of the literature is that this peculiar larval form is shared in common by no less than three of the echinoderm classes. Furthermore, it is invariably associated with a yolkmass in the egg. For this distinctive larval form I propose the term “Vitellaria”—or “yolk-larva”; it can only be considered as an independent sequence.
The general characters which distinguish this larval type are as follows:—The body is simply organised, having no pairs of larval arms, or other projecting organs. It is cylindrical or barrel-shaped, and opaque owing to the presence of yolk-material in the tissues. It is free-swimming, and is provided with rings of cilia. These bands are variable in number, but their general disposition is the same. There may be a larger or smaller tuft of cilia at the anterior end. They are commonly deeply pigmented. In Figs. 21–24 four larvae of this type are shown together; their common pattern is obvious, though they are drawn from three different classes of echinoderms. Indeed, these larvae, though belonging to Holothurians, Ophiuroids and Crinoids, have far more in common than have many of the Plutei or Bipinnariae. The Bipinnaria of Asterias, for example, is widely different from the great Bipinnaria of Luidia.
Hitherto in the literature these various forms have been considered separately. The Vitellaria is found among the Crinoids, and from that group no other larva is as yet known. Of Ophiuroidea, species of Ophiura are known to possess the larva in both America
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Figs. 21–24.—Examples of echinoderm “yolk-larvae.” Figs. 21 and 22, of Holothuroidea (Cucumaria and Labidoplax); Fig. 23, of Ophiuroidea. (Ophiura brevispina); Fig. 24, of Crinoidea (Antedon bifida). Fig. 21, after Selenka; Fig. 22, after Dawydoff; Fig. 23, after Grave; Fig. 24, after Seeliger.
and Europe. It is the same as “The Worm-like Larva” of Muller, a fact first pointed out by Grave. Among the Holothuroidea, some species possess only the Vitellaria (“Barrel-shaped larva” or “pupa”), while others possess an Auricularia which later becomes a Vitellaria.
In view of the diverse internal organogeny it is impossible to regard the Vitellaria as representing any ancestral type. It seems to be a generalised yolk form developed independently by the various classes under some special circumstances of which we have no information. The circumstances, whatever they may be, are undoubtedly connected with the yolk mass common to them all, but why the larva should assume such a typical annulated form in each case is a problem. It is evidently an example of convergent evolution affecting the larval forms without changing the adult—that is, without leaving its impress upon phylogeny. The process by which the Vitellaria is developed is therefore yet another example from the echinoderms of the mechanism termed by de Beer “Clandestine Evolution”.