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Embryos and larvae of an isocrinid sea lily, Metacrinus rotundus, are described by scanning electron microscopy. Around hatching (35 h after fertilization), the outer surface of the gastrula becomes ubiquitously covered with short cilia. At 40 h, the hatched swimming embryo develops a cilia-free zone of ectoderm on the ventral side. By 3 days, the very early dipleurula larva develops a cilia-free zone ventrally, densely ciliated regions laterally, and a sparsely ciliated region dorsally. At this stage, the posterior and anterior ciliary bands first appear: the former runs along a low ridge separating the densely from the sparsely ciliated epidermal regions, while the latter is visible, at first discontinuously, along the boundary between the densely ciliated lateral regions and the cilia-free ventral zone. In the late dipleurula larva (5 days after fertilization), the anterior and posterior loops of ciliary bands are well defined. The transition from the dipleurula to the semidoliolaria larva occurs at 6 days as the posterior loop becomes rearranged to form incompletely circumferential ciliary bands. The larva becomes competent to settle at this stage. The arrangement of the ciliary bands on the semidoliolaria is maintained during the second week of development, while the larva retains its competence to settle. The larval ciliary patterns described here are compared with those of stalkless crinoids and eleutherozoan echinoderms. The closest morphological similarities are between M. rotundus and the basal eleutherozoan class Asteroidea.
Fig. 1. Scanning electron microscopy of the early development stages of the ciliary bands in the embryos and larvae of M. rotundus. ─A. An embryo 35 h after fertilization just before hatching. The fertilization envelope has been removed as described in Materials and Methods. The vestigial fertilization envelope remains in the bottom left-hand corner of the picture. ─B. An embryo at the predipleurula swimming stage 40 h after fertilization soon after hatching. ─C. Enlargement of the rectangle in B, showing the boundary between the ciliated zone and cilia-free zone. ─D. An embryo at the predipleurula swimming stage 2 days after fertilization. ─E. A larva at the very early dipleurula stage 3 days after fertilization viewed from the right-ventral side. Arrows and arrowheads in E–H show the ciliary bands and a continuous low ridge, respectively. Tandem arrows in E, G, and H show fragment of ciliary band formed at the boundary between the densely ciliated lateral areas and the cilia-free zone. ─F. A part around the posterior end of the same larva shown in E viewed from the posterior-ventral side. ─G. A larva 3 days after fertilization viewed from the right-dorsal side. This specimen is a different individual from that in E. ─H. A larva at the early dipleurula stage 3.5 days after fertilization viewed from the right-dorsal side. Scale line in A, D, and E (applicable also to G and H) = 100 μm. Scale line in B and F = 50 μm. Scale line in C = 20 μm. AB, anterior bulge; AF, anal field; BP, blastopore; C, cilia; CF, cilia-free zone; CZ, ciliated zone; FF, frontal field; HP, hydropore; ND, narrow dent; PB, posterior bulge; VF, vestigial fertilization envelope; PE, posterior end; VI, vestibular invagination.
Fig. 2. Dipleurula larva of M. rotundus 5 days after fertilization. ─A. Diagram of the larva from the left side (ciliary bands shown by hatching and other heavily ciliated regions by stippling). The diagram indicates the anterior-posterior axis and oral-aboral (ventral-dorsal) axis. Abbreviations, in clockwise order from the top: CT, ciliary tuft; AL, anterior loop of ciliary band; HP, hydropore; PL, posterior loop of ciliary band; CBP, closed blastopore; VI, vestibular invagination; AP, adhesive pit; AC, adhesive concavity. ─B. A perspective view showing the total pattern of two ciliary bands formed along the low ridges and five pairs of projecting ridges along the bands in the larva. The figure is viewed from the left ventral side of the larva with the anterior end at the top. Abbreviations, in clockwise order from the top: va, ventral anterior ridge; da, dorsal anterior ridge; dp, dorsal posterior ridge; lp, lateral posterior ridge; PL, posterior loop of the ciliary band; vp, ventral posterior ridge; AL, anterior loop of the ciliary band. ─C. SEM showing the ventral side, including the adhesive concavity (single arrow) and closed blastopore (tandem arrow); boxed region enlarged in I. ─D. SEM showing the left side; vestibular invagination, closed blastopore, and hydropore indicated, respectively, by an arrowhead, tandem arrow, and single arrow. ─E. SEM showing the dorsal side; hydropore is indicated by a single arrow. ─F. SEM showing the right side. ─G. SEM showing the animal pole with an ciliary tuft (arrowhead) and adhesive concavity (single arrow). ─H. SEM showing the posterior end. ─I. Enlargement of the rectangle in C, showing part of the adhesive pit (arrowhead) and vestibular invagination (single arrow). Scale line in C (applicable also to D–H) = 100 μm. Scale line in I = 50 μm.
Fig. 3. Semidoliolaria larva of M. rotundus 6 days after fertilization. ─A. Diagram of the larva from the left side (ciliary bands shown by hatching and other heavily ciliated regions by stippling). Abbreviations, in clockwise order from the top: AL, anterior loop of ciliary band; HP, hydropore; CBP, closed blastopore; VI, vestibular invagination; AP, adhesive pit; AC, adhesive concavity. The posterior ciliary band has become rearranged into segments that partially or completely encircle the body. ─B. A perspective view showing the total ciliary band pattern in the larva. The larva is viewed from the left ventral side with the anterior end at the top. ─C. SEM showing the ventral side, including the adhesive concavity (single arrow) and closed blastopore (tandem arrow); boxed region enlarged in I. ─D. SEM showing the left side; vestibular invagination and hydropore indicated, respectively, by an arrowhead and a single arrow. ─E. SEM showing the dorsal side; hydropore indicated by a single arrow. ─F. SEM showing the right side; vestibular invagination indicated by an arrowhead. ─G. SEM showing the animal pole with the adhesive concavity (single arrow). ─H. SEM showing the posterior end. ─I. Enlargement of the rectangle in C, showing part of the adhesive pit (arrowhead) and vestibular invagination (single arrow). Scale line in C (applicable also to D–H) = 100 μm. Scale line in I = 20 μm.
Fig. 4. Perspective views showing the total pattern of ciliary bands and five (left) or six (right) pairs of projecting ridges along the bands in the asteroid bipinnaria (left) and holothurian auricularia (right) larvae (based on Lacalli 1993). The bipinnaria is viewed from the left ventral side, and the auricularia is viewed from the right-ventral side with the anterior end at the top. Abbreviations, in clockwise order from the top: pr, pre-oral ridge; ad, anterior-dorsal ridge; md, mid-dorsal ridge; po, postoral ridge; pd, posterior-dorsal ridge; pl, posterior-lateral ridge; PT, postoral band; PR, pre-oral band.
Fig. 5. Perspective views showing the transition of ciliary band patterns in M. rotundus from the dipleurula to the semidoliolaria type. Each colored region in the dipleurula ciliary bands forms the region of the corresponding color in the semidoliolaria bands after transition.
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