A treatise on zoology . f the water-vascular system. The development of Echinoidea has been studied by J. Miiller(1852), Agassiz (1864), Metschnikoff (1869), Bury (1889), andmany others. The results are summarised by Theel in his admir-able account of the development of Echinocyamus pusillus (1892).Up to the stage corresponding to the Dipleurula no importantdivergences are manifest. The peculiarities of the ensuing meta-morphosis appear due to the extreme development of a free-swimming Pluteus (Fig. XIV.). At an early stage there is aninvagination (am) of the ectoderm on the left side between
A treatise on zoology . f the water-vascular system. The development of Echinoidea has been studied by J. Miiller(1852), Agassiz (1864), Metschnikoff (1869), Bury (1889), andmany others. The results are summarised by Theel in his admir-able account of the development of Echinocyamus pusillus (1892).Up to the stage corresponding to the Dipleurula no importantdivergences are manifest. The peculiarities of the ensuing meta-morphosis appear due to the extreme development of a free-swimming Pluteus (Fig. XIV.). At an early stage there is aninvagination (am) of the ectoderm on the left side between thebases of the ventral and dorsal posterior processes of the inner end of this sac grows towards the left hydrocoel,Avhile its opening nearly or quite closes (Fig. XY. 1). The fiveprimitive lobes of the hydrocoel grow up into the floor of thissac (Fig. XV. 2), which thus serves as a kind of amnion in whichthe young sea-urchin is formed (Fig. XV. 3), until the size of the i6 ECHINODERMA—GENERAL DESCRIPTION. Fig. XIV. PZw<eu5 of EcTii ftocT/a wins (after Theel).About 75 times nat. size. Explanation of letters to and XV.—, anterior coelom ;, anterior dorsal arm; am, am-niotic invagination ; , anteriorventral arm; Tic, hydrocoel; hy,hydropore ; , left posteriorcoelom ; 0, mouth ; oe, oesophagus ;p, podia ; ?pd, posterior dorsal arm ;, posterior ventral arm; ,right posterior coelom ; s, spines ofEchinoid ; sp, spicules of Pluteus :sp, the same being absorbed ; st,stomach.
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Keywords: ., bookcentury1900, bookdecade1900, bookpublishe, booksubjectzoology
