. The Cambridge natural history. Zoology. 6oo ECHINODERMATA BLASTOIDEA by the extension of the side-plates of the radial vessel, and the whole group of folds has a common opening near the mouth; indeed, in the highest form there is one common " spiracle " for the two groups of folds in an interradius, which in one interradius is confluent with the anus. The hydrospires, when they reach this form, irresistibly recall the genital bursae of Ophiuroidea (Fig. 214, p. 490), and very possibly served the same purpose. Eeviewing the whole group of the Pelmatozoa, we see that in the Cambrian
. The Cambridge natural history. Zoology. 6oo ECHINODERMATA BLASTOIDEA by the extension of the side-plates of the radial vessel, and the whole group of folds has a common opening near the mouth; indeed, in the highest form there is one common " spiracle " for the two groups of folds in an interradius, which in one interradius is confluent with the anus. The hydrospires, when they reach this form, irresistibly recall the genital bursae of Ophiuroidea (Fig. 214, p. 490), and very possibly served the same purpose. Eeviewing the whole group of the Pelmatozoa, we see that in the Cambrian they begin with the extremely primitive Thecoidea and Carpoidea, together with some obscure forms which, com- bining a stem with pentamerous symmetry in the calyx, are sup- posed to be the forerunners of the Crinoidea. In the Lower Silurian or Ordovician the two groups of Fig. 2/9.âGranatocrinus noriwodi. A, ,, ^ , . , i j_i ⢠view of whole animal; B, seotiou of the Lystoidea make their appear- radius; c, an isolated linger, hyd, ance, possibly independently de- Hydrospire ; I, lancet plate; pmn, â â , \~, â ^ finger; j>.p, covering plate; R and veloped ±rom either Garpoidea or Z) both signify radial plate. (After primitive Crinoidea, which. m Zittel.) this period are present in unmis- takable form. In the Upper Silurian the Blastoidea appear, distinguishable from the most regular Cystoidea only by their hydrospires. It seems practically certain that they were developed from Cystoidea, and we follow Jaekel in believing that they arose from Dichoporita. The Carpoidea do not extend beyond the Ordovician, and by the end of the Carbo- niferous period Cystoidea and Blastoidea die out, leaving only the Crinoidea, which at that period were at their maximum development. From the Carboniferous to the present day the Crinoidea have continually decreased, leaving in recent seas, as sole representatives of the Pelmatozoa, only the few forms described at the beginning of this c
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
