A treatise on zoology . al cavity is large inproportion to the thickness of the arms, and is enclosed by thinflexible .walls. Of^these three genera, Saccocoma is the most special-ised, and is supposed by Jaekel (1893) to have been pelagic, livingin swarms. Uintacrinus, with its extraordinarily long and movablearms, may also have been pelagic. The genera of this third group,although of origin as diverse as those in the other groups, resembleone another in the presence of a central, pentagonal, apical in Saccocoma may be the fused basals ; in Uintacrinus and Marsu-pites it represents
A treatise on zoology . al cavity is large inproportion to the thickness of the arms, and is enclosed by thinflexible .walls. Of^these three genera, Saccocoma is the most special-ised, and is supposed by Jaekel (1893) to have been pelagic, livingin swarms. Uintacrinus, with its extraordinarily long and movablearms, may also have been pelagic. The genera of this third group,although of origin as diverse as those in the other groups, resembleone another in the presence of a central, pentagonal, apical in Saccocoma may be the fused basals ; in Uintacrinus and Marsu-pites it represents neither basals nor infrabasals, but may be theproximale, or the supposed distal columnar plate (dorso-central),or a new supplementary plate. It is safest to call it centrcde. Another curious modification, perhaps connected with a free-floating existence, was presented by the root of swelled out into a hollow, chambered, balloon-like body,referred by Barrande to an independent class of Echinoderms under. 136 THE CRINOIDEA the name Lobolithus, and described by Hall as a float, which hecalled Camarocrinus. As regards the internal organs of the crinoid not much can besaid. The most remarkable modifications are those aflecting theGut. In most recent crinoids this makes a simple dextral coilaround the thecal cavity, from central mouth to eccentric mouth may be slightly shifted anteriorly by increase in sizeof the anus, or by the anal tube coming to occupy the centre ofthe tegmen, as in Batocrinus, or even to pass beyond it towardsthe anterior margin, as in Siphonocrinus (p. 199). But the mouthremains in the axis of the coil, and such forms are called endo-cyclic. In Adinometra (p. 196) the gut winds in the same way,but instead of issuing immediately the first coil is completed,it continues to coil, not however around the axis of themouth but around the axis of the anus. The mouth, with itsannular accompaniments, therefore lies between the outer coil andthe
Size: 1338px × 1868px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, bookpublishe, booksubjectzoology
