OCR Text |
Show 1884.] SPECIES OF OREASTER. 59 best adapted for arriving at some clearer ideas as to the relations of the species among themselves, and the history of an ancient generic group. First of all, we may well expect some differences in external appearance, in the relations of the greater to the lesser radii, and in the width and proportions of the arms, the moment we know that specimens may attain to a spread of 400 millims. or more, or attain to a height of 120 millims. ; while, however, we shall find growth-differences in some, we shall in others, such as 0. nodulosus, be struck rather by the constancy of proportions in the post-larval stfiges of devolopment. Our experience of other long-armed forms, such as Linckia or Ophidiaster, might lead us to ask, Does Oreaster, like these genera, tend to lose its arms, and does it, like them, reproduce itself asexually, or exhibit any other mode of heteractinism 1 Heteractinic conditions are exceedingly rare among Oreasters, and it follows therefore that the dangers to which the species are exposed are slight, its skeletal structures are very strong, or its power of active or passive defence very great. As to the danger we know but little ; as to the skeletal structure, we know that it is eminently reticulated on the upper surface ; and, now, as to the organs of defence, we know that many of the species are well provided with marginal or dorsal spines of considerable length, and that, in some cases, the proper ventral plates are very spinous. To a certain extent these spines present us with very definite characters. W e can, for example, always safely discriminate between O. lincki and O. nodosus, by examining the free ends of the arms, the sides of which in the former are constantly, and in the latter are never, provided with outstanding spines. So, again, the species described by Perrier under the name of 0. alveolatus may, as it seems, be certainly separated from O. lincki, owing to the fact that the infero-marginal plates bear well or fairly developed spines. For the purposes of this investigation we shall, perhaps, do well to study attentively one of the species of the genus in which the spinous armature is well developed-O. armatus : three specimens, in which Ris respectively equal to 23*5, 37, and 85 millims., have at the proximal end of the middle line of each arm a spine measuring 1, 3, and 14 millim. respectively. In (a) the marginal plates rarely exhibit any break in their regular granulation ; when they do so, we find a naked papilla just projecting beyond the level of the granular investment; no spines are developed in the spaces between the middle lines of each ray; in the centre of the disk is a spine which is about equal in size to those which mark the end of the arms; the other spines along the middle line are nothing more than mere papilliform processes. On the ventral plates spines are developed indeed, but they are as yet only rounded projections which are just beginning to be distinguishable from the investing granules of the ossicles which bear them. In the next specimen Q3) the spines of such marginal plates as |