. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 106 A. L. PARKS ET AL. Figure 8. Asthenosoma ijimai lacks a vestibule. 6 nm Alizarin red and methylene blue stained section of a 101-h A ijimai larvae showing one psuedo arm (P) and a developing tube foot (TF). Embryos of sev- eral stages (35, 45, 56, 75, and 101 h) were serial sectioned and no evi- dence of a vestibule was found at any stage across the developing adult rudiment. Scale bar = early development in lecithotrophic echinoids of three, independently evolved euechinoid lineages [Echinome- tridae, Echinothuriidae, T
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 106 A. L. PARKS ET AL. Figure 8. Asthenosoma ijimai lacks a vestibule. 6 nm Alizarin red and methylene blue stained section of a 101-h A ijimai larvae showing one psuedo arm (P) and a developing tube foot (TF). Embryos of sev- eral stages (35, 45, 56, 75, and 101 h) were serial sectioned and no evi- dence of a vestibule was found at any stage across the developing adult rudiment. Scale bar = early development in lecithotrophic echinoids of three, independently evolved euechinoid lineages [Echinome- tridae, Echinothuriidae, Temnopleuridae (Raff, 1987)], they are likely to represent functional adaptations to. or are consequences of, a change in developmental mode. This hypothesis is strengthened by the fact that these fea- tures are not characteristic of planktotrophic develop- ment in either cidaroids or euechinoids. Skelctogenic mesenchyme cells and expression of msp!30 We can distinguish three distinct classes of mesen- chyme cells in the P. parvispinus larva: skeletogenic and nonskeletogenic mesenchyme (msp!30 positive and negative) cells in the blastocoel, and pigment cells in- serted in the ectoderm. Indirect developing cidaroid and euechinoid embryos contain these same three classes of mesenchyme cells (Gibson and Burke, 1985; Wray and McClay, 1988). One of the most striking features of lecithotrophic de- velopment in Heliocidaris erythrogramma, a euechi- noid, is the elimination of the larval pattern of skeleton formation and its replacement by an accelerated adult skeleton assembly. The expression pattern of msp!30, a protein produced in larval and adult spicule-forming cells of echinoids, is also altered in direct developing echinoids (Parks et ai, 1988). In planktotrophic euechi- noid larvae, mspl30 is first expressed at, or some time before, synthesis of the larval skeleton (Wray and Mc- Clay, 1989). In H. erythrogramma, mspl30 expression is delayed relative to that in planktotrophic lar
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