. Elementary text-book of zoology. Zoology. ISIV1UM—TRIVIl'M. 269 wliile, in Spatangidce, both poles, or only the oral pole, are shifted in the direction of the unpaired radius, and become eccentric (tig. 208). Only a few of the regular EcJiiiiv<l<'rni«t<i have the means of loco- motion on all the live rays, and seldom then along the whole length of their meridians; far more frequently the area surrounding the oral pole becomes with regard to the position during movement the ventral surface; it is flattened and mainly or entirely possesses the organs of locomotion (am- bulacral surfac
. Elementary text-book of zoology. Zoology. ISIV1UM—TRIVIl'M. 269 wliile, in Spatangidce, both poles, or only the oral pole, are shifted in the direction of the unpaired radius, and become eccentric (tig. 208). Only a few of the regular EcJiiiiv<l<'rni«t<i have the means of loco- motion on all the live rays, and seldom then along the whole length of their meridians; far more frequently the area surrounding the oral pole becomes with regard to the position during movement the ventral surface; it is flattened and mainly or entirely possesses the organs of locomotion (am- bulacral surface). These re- la tions always obtain among the irregular Echinoder- 'iiH/f't which do not move indifferently in the direction of all live rays, but princi- pally in that of the unpaired one. In these animals the mouth, and therewith the oral pole, being pushed to- wards the anterior edge, the two posterior radii (bivium) seem principally concerned in the formation of 'the ventral surface (Spa- tangidce). It is otherwise in the case of the cylindrical HolotJini'lans. Their mouth and anus preserve the nor- mal position at the poles of the elongated axis, and the body is not unfrequeiitly compressed in the direction of the axis in such a manner that three radii (tririuiii) with their organs of locomotion are placed on the foot-like ventral surface. On the body of these Ilnlotli u ri« as one unpaired and two paired radii can be distinguished, only in this case the unpaired radius with its inter-radius marks, not the anterior and posterior, but the dorsal and ventral surfaces. In many Echinoderms (Echinoidea) the oblate spheroidal form is the prevalent one. The principal axis appears shortened, the apical pole may be either pointed or flattened, and the ventral half is. FIG. •im. — CiirttnKtrhi with extended dendritically branched tentacles (T). Af, ambulacra! Please note that these images are extracted from scanned page images that may have been digitally enhanced fo
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
