Revison of the Echini Volume 1-2

Revison of the Echini Volume 1-2

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This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1874 edition. Excerpt: ...showing in their young stages that they have but a small number of pores for each ambulacral plate, which places them among the Oligoporidae; but, owing to the independent growth of the plates of the poriferous zone in older stages, they seem to belong to the Polyporidae. The changes which the tubercles undergo in their growth are very marked; it constantly happens that primary tubercles, in way of formation, become resorbed by the rapid growth of the test; at least, I can account for the variation of the abactinal part of the interambulacral areas in no other way (compare f. 14, 16', 18, PZ. ). Young specimens of such allied species as A. punctulata and A. pustulosa can readily be recognized by the differences of the abactinal system, which is quite pentagonal in young specimens of A. pustulosa (Pl. V. f. 19, 20), while the position of the ocular plates relatively to the genital plates presents in this system a totally diiferent appearance from A. punctulata (PZ. V. f. 1.4, 18), where the ocular plates project beyond the line of the genital plates. The variations which occur at diiierent stages of growth of this species show how difliclllt it must be to distinguish the various species which have been described at different times, many of which undoubtedly rest upon no more substantial basis than variations due to peculiarities of young stages which have been more or less persistent. The adult (when examining a large series) shows no less liability to variation in the height of the coronal plates, the size of the tubercles, their number upon the sides of the test, and the relative number of the larger and smaller tubercles, as well as the spines they carry; the sculpture even of the plates of the abactinal system, ...show more

Product details

  • Paperback | 86 pages
  • 189 x 246 x 5mm | 168g
  • Rarebooksclub.com
  • United States
  • English
  • black & white illustrations
  • 1236831519
  • 9781236831514