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Show 500 PROF. F. J. BELL ON THE GENUS ASTERIAS. [May 3, of rare species more exact and definite information than has yet been afforded. But little light is thrown upon the subject by a reference to the characters which obtain in Brisinga, owing to the extremely generalized character of that important form ; the adambulacral spines are, according to the careful description of G. O. Sars, arranged in the adult in three longitudinal rows, of which those of the outermost row are distinctly the longest; " in very young specimens," however, it is to be noted that the " two innermost are yet undeveloped, so that there is only a single longitudinal row of furrow-spines along the middle of the adambulcral areas." Although there is this tendency to a disposition of the adambulacral spines in one or two rows, there are (a) cases, e. g. A. panopla, Stuxberg, in which there are three rows, and (ft) specimens of the common and widely-spread A. rubens and of other species which do not always exhibit a constant regularity : the spines may be set more or less distinctly along three planes ; but it is rarely that three spines are found on one and the same plate; and it is clear that there is a tendency to the development of a single continuous series, arranged as nearly as possible in a perfectly straight row. If, further, we consider how extremely short is the longitudinal axis of the ossicles, and the size of the spines that are placed on them, and that the packing of the spines does nevertheless diverge but little into any thing whatsoever of a zigzag arrangement, we are, I think, justified for the present in not enforcing any such further subdivision as would be expressed by such terms as isacanthid and anisacanthid? There is a considerable body of fact which would^ justify such a division ; but there is need of further study and fuller information to enable us to speak definitely as to the permanency of any real anisa-canthid arrangement; the species in the following lists, which are distinguished by an asterisk sign are those in which this irregularity is most conspicuous. III. Primary Groups of the Species of Asterias. A. HETERACTINIDA. I. POLYPLACIDA. i. MONACANTHIDA. calamaria. tenuispina. ii. DlPLACANTHIDA. *acutispina. polyplax. capensis. II. MONOPLACIDA. i. MONACANTHIDA. gelatinosa. perrieri. gemmifer. rodolphi. katherina?. 1 These terms would, of course, express the difference between forms in which the spines are the same in number on all the plates, and those in which the spinea are unequally distributed. |