. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. 2G3 conditions which admit of the body being divided into two exactly symmetrical halves. Even these planes do not exactly fulfil these con- ditions, since the re- maining organs are not strictly symmetrical in regard to such a plane. Very frequently one of the rays differs in size from the others, and then we have an irregu- larity in the external form of the Echinoderm, which renders the bi- lateral symmetry visible even from the exterior. The pentamerous body of the Echin
. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. 2G3 conditions which admit of the body being divided into two exactly symmetrical halves. Even these planes do not exactly fulfil these con- ditions, since the re- maining organs are not strictly symmetrical in regard to such a plane. Very frequently one of the rays differs in size from the others, and then we have an irregu- larity in the external form of the Echinoderm, which renders the bi- lateral symmetry visible even from the exterior. The pentamerous body of the Echinoderm may become bilateral, the plane of the unpaired ray forming a median plane, on each side of which two pairs of equal rays are repeated. We can distinguish an upper sur- face (apical pole) and an under (oral pole), a right and left side (the two paired rays and their inter-radii), an anterior end (un- paired radius) and a posterior (unpaired inter-radius). In irregular Sea-urchins, the bilate- rally symmetrical form is more strongly marked. Not only is the unpaired radius of abnormal size and form, and not only are the angles at which the principal ray and the accessory rays cut Fro. 207.—CJypsaster rosaceut, from the dorsal side. The madreporic plate is situate in the centre and is sur- roundad by five genital pores and by the five-leaved rosette. The unpaired radius is directed forwards. At the side is the median portion of the ventral sur- face. O, mouth; A, Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Claus, Carl, 1835-1899; Sedgwick, Adam, 1854-1913; Sinclair, F. G. (Frederick Granville), b. 1858. London, Swan Sonnenschein & co.
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