. A textbook of invertebrate morphology [microform]. Invertebrates; Morphology (Animals); Invertébrés; Morphologie (Animaux). lil 1« 672 INVERTEBRATE MORPHOLOGY. include all the plates of the apical system, and at the same time the anal opening may leave its position near the centre of the apical system and become situated in the interradius AB, either at the margin of the flattened disklike test, or even on its oral surface. A marked bilaterality of form is thus de- veloped, which may become still more pronounced by a mi- gration of the mouth away from the centre of the oral surface along the
. A textbook of invertebrate morphology [microform]. Invertebrates; Morphology (Animals); Invertébrés; Morphologie (Animaux). lil 1« 672 INVERTEBRATE MORPHOLOGY. include all the plates of the apical system, and at the same time the anal opening may leave its position near the centre of the apical system and become situated in the interradius AB, either at the margin of the flattened disklike test, or even on its oral surface. A marked bilaterality of form is thus de- veloped, which may become still more pronounced by a mi- gration of the mouth away from the centre of the oral surface along the line of the radius U, which at the same time be- comes more or less altered in size and form, and consequently dissimilar to the other radii (Fig. 263). In these cases it is possible to recognize in addi- tion to oral and aboral surfaces anterior and posterior poles and a right and left side, the median line of the body pass- ing in front through the radius D and posteriorly through the interradius AB. Three of the radii, 6', D, and J?, thus lie in the anterior half of the body, and for descriptive purposes these have been termed the trivium, while the two posterior ones, A and B, constitute the bivium. The mouth, which is usual- ly situated in the centre of the aboral surface, is surrounded by ah area, the peristome, which has imbedded in it only a few scattered calcareous plates and consequently possesses a somewhat leathery consistency. An oral system of plates cannot be distinguished in adult Echinoids. The marked bilateral symmetry referred to above as occurring in cer- tain Echinoids is undoubtedly a secondary condition, those forms in which the mouth is central and the anus approximately so, and whose bilaterality is indicated only by the madreponform tubercle, being, there is every reason to believe, the most primitive. The bilaterality cannot be regarded as a reversion to the more primitive symmetry of the larva, since in the. Fig. 263.—A PETALosTfCHous Echi- NOi
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Keywords: ., bookcentury1800, bookdecade1890, bookpublishernewyo, bookyear1894
