Studies in the Embryology of the Echinoderms

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Abstract
Not only has it been shown in the foregoing pages that many skeletal plates are developed in the bilateral larva (Dipleurula), and that they bear a definite relation to the body-cavities, but the discovery that the terminals lie on the left side enables us to establish a typical bilateral form from which all the conditions found in existing larvse may have been derived; this typical form has five radial and five interradial plates on each side, in definite relation to the body-cavities, as shown in the following Table, in which are also given the names by which the plates in question are usually known. Position: Right Enterocœl. Radial, Interradial. Name: Primary Radials, Basals Position: Left Enterocœl. Radial, Interradial. Name: Terminals, Orals In Ophiurid Plutei the ten radial plates (primary radials and terminals) and one of the orals (madreporite) are early developed ; but the basals and most of the orals do not appear till the pentagonal stage is reached. In Crinoids, on the other hand, the ten interradials (basals and orals) are the first to appear, while the primary radials arise late; no terminals have yet been recognised, but now that we know where to look for them it is not impossible that they may be discovered. In Asterids the terminals are usually the first plates to show themselves, though in Asterina the basals arise simultaneously with them ; in other forms the basals (except the madreporite) and the primary radials are late in appearing, and it is not yet certain whether the orals are ever developed. In Echinids the basals appear early, but we know nothing at present of the primary radials, terminals, and orals; the ocular plates have usually been identified as primary radials, but some regard them as terminals; embryology alone can decide this question, and at present my material is not sufficient for it; I am, however, strongly disposed to believe that the oculars are terminals, and that the primary radials are entirely absent. The definite relation borne by the plates to the body-cavities is a fact of great morphological importance, and while it is absolutely opposed to Barrois's statement that all the skeletal plates of Ophiurid, Echinid, Asterid, and Crinoid larvae are developed round the right enteroccel, it also throws considerable doubts on Semon's sweeping assertion (28, p. 282) that no homologies are to be found between the primary plates in the different groups. Another point of considerable morphological importance receives great light from the study of the development of the calcareous plates. If we look at the lateral views of Ophiurid and Asterid larvse (figs. 2 and 18) we shall see that the plates on each side may be regarded as forming a longitudinal series dorsal to the alimentary canal; since those plates which are apparently ventral do not reach as far forward as the anus, and may be conceived to have reached their present position in connection with a general curvature of the body. The arrangement, in fact, strongly suggests segmentation, but I cannot discuss in the present paper whether the pentamerism of the adult Echinoderms arose in this way in the skeleton, or whether it first made its appearance in the hydrocoel; the latter appears to me more probable. It is further evident from figs. 2 and 13 that, taking the terminals as marking the radii, the mouth, anus, and water-pore are at this stage all in the same interradius. It may be asked, Why should the terminals rather than the water-vascular pouches be taken to mark the radii? The fact is that during metamorphosis the hydrocoel undergoes such extraordinary changes of position that it is doubtful whether any reliance can be placed on the position of its pouches in the larva as indicating any permanent relations to the rest of the body; at any rate, it is certain that the water-pore is much more constant in its relations to the mouth and anus than to the water-vascular pouches. Thus we know that in Asterina (17) the most anterior tentacular pouch (in front of the water-pore) is eventually embraced by that terminal which seems to be morphologically the most posterior, i. e. the one just behind the anus in fig. 13. It is possible that this is also the case in Bipinnaria, but if so, it is the tentacular pouch alone which shifts its position; for in fig. 15 we see that, after the connection of the terminals with their respective water-vascular pouches, the mouth, anus, prseoral lobe, water-pore, and watertube still lie in one and the same interradius (see also 2 and 1, pl. vii, fig. 6). The same figure also indicates what I shall prove more fully in a future paper, that the left anterior enterocœl becomes the so-called "Schlauchfrmiger Kanal" of the adult. The Ophiurids are still more remarkable than Asterina in the behaviour of their hydrocoel; as already pointed out, the whole hydrocoel is pushed forward and round the oesophagus in such a way that the tentacular pouch immediately behind the water-tube (most posterior in fig. 2) unites with the most anterior terminal, while the pouch ju3t in front of the water-tube unites with the terminal which is nearest the anus. In this case it is clear that to distinguish the anterior and posterior radii by means of the tentacular pouches, before they have selected, so to speak, their respective terminals, would only involve us in confusion. In a previous paper (7, p. 294) I imagined that the arrangement found in Antedon, in which the mouth and anus are actually in the same interradius in the adult, was arrived at by a secondary shifting of the anus. This appears to be Ludwig's idea (14, p. 54), and is also advocated by Barrois for Antedon (5, p. 638). Now, however, I am compelled to regard this position of the anus as primitive, though of course it is still possible that in this particular group it may have been secondarily derived from such a condition as is found in Asterids. Of the view here advanced, that the mouth, anus, and waterpore...

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