. Text-book of embryology. Embryology. XVI ECHINODEEMATA 535 In this form the first five lobes of the hydrocoele give rise to the radial canals, and the primary buccal tentacles arise from their bases. Now in Synapta the radial canals disappear in the adult, they are, in a word, vestigial functionless structures â , but in Cucumaria they persist and give rise to numerous paired tube feet. Therefore the mode of their development in Synapta is secondary, the method in Cucumaria typical and primary, and this typical and primary method is the same as that which oIdtains in other groups of Echinode
. Text-book of embryology. Embryology. XVI ECHINODEEMATA 535 In this form the first five lobes of the hydrocoele give rise to the radial canals, and the primary buccal tentacles arise from their bases. Now in Synapta the radial canals disappear in the adult, they are, in a word, vestigial functionless structures â , but in Cucumaria they persist and give rise to numerous paired tube feet. Therefore the mode of their development in Synapta is secondary, the method in Cucumaria typical and primary, and this typical and primary method is the same as that which oIdtains in other groups of Fig. 398.âThe fully developed Auricnlavia larva of ^ynapfa digilata viewed from the ventral surface and from the side. (After Bury.) A, Auricularia viewed from the ventral surface. The darker regions of the longitudinal ciliated band indicate the fragments into which it will break at the metamorphosis. B, Auricularia viewed from the left side. Letters as in previous figure. In addition, I-V, the fragments of the ciliated band which will give rise to the transverse ciliated bands of the pupa. OR, the fragments of the ciliated band which will give rise to the ring surrounding the mouth of the pupa. li-Sj, the lobes of the hydrocoele which will give rise to the first five buccal tentacles. 1-4, lobes of the hydrocoele giving rise to radial canals, a, anus; ax, left anterior coelom; , antero-dorsal arm ; , adoral band of cilia; hy, hydrocoele; int, intestine ; intermediate dorsal arm; Lnerv, larval nervous system of Semon ; , pore-canal; , postero-dorsal arm; , arm ; , post-oral arm; peri- oral coelom ; 2y', prae-oral arm ; , stone-canal. The mesenchyme in Synapta has by this time formed a loose reticulate tissue spanning the blastocoele. In the postero-lateral process certain of the mesenchyme cells have given rise to typical calcareous structures which enable us to diftinguish the larva of Synapta digita
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Keywords: ., bookcentury1900, bookdecade1910, bookpubli, booksubjectembryology
