. A manual of zoology. Zoology. IV. ECniNOIDEA. 513 Class IV. Echinoidea (Sea Urchins). The structure of the sen urchins is best understood in the spherical forms (figs. 328, 330). Moutli and anus lie at opposite poles of the main axis, each open- ing immediately surrounded hy areas covered by calcareous plates, the arrangement of which varies with the family. Around the anus is the periproct, around the mouth the peristome, the latter bearing sphasridia and in the Echinoids five pairs of interambulacral gills. Be- tween peristome and periproct the body wall is composed of calcareous plates, w
. A manual of zoology. Zoology. IV. ECniNOIDEA. 513 Class IV. Echinoidea (Sea Urchins). The structure of the sen urchins is best understood in the spherical forms (figs. 328, 330). Moutli and anus lie at opposite poles of the main axis, each open- ing immediately surrounded hy areas covered by calcareous plates, the arrangement of which varies with the family. Around the anus is the periproct, around the mouth the peristome, the latter bearing sphasridia and in the Echinoids five pairs of interambulacral gills. Be- tween peristome and periproct the body wall is composed of calcareous plates, which, except in the Echino- thuridas, are immovably united. Aside from the extinct Pahechei- noidea the plates are arranged in twenty meridional rows, or, more accurately, in ten double rows, two rows being always intimately associated together. Five of these double rows are ambulaci' Fro.!i-2».—rirliipIciirvxflork!ainis.* (After Agassiz.) Aboral view, the spines removed to sliow the ambulacral (a) and (/)) interambulacral areas, end- ing respectively in the ocular and genital plates ; in the centre the four plates of the 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 Hertwig, Richard, 1850-1937; Kingsley, J. S. (John Sterling), 1854-1929. New York, H. Holt and Company
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectzoology, bookyear1902
