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Show 414 PROF. F. J. BELL ON THE ECHINOMETRIDsE. [Mar. 15, growth of the pore-plates, the distinction between them altogether breaks down ; whether that distinction be physiological or morphological is, then, an unnecessary question. Coming next to the absolute distinctness of the groups as indicated by the number of the pairs of pores, we are met, first of all, by the considerations which surround the vexed question of the value of any delimitation by the absolute use of definite numbers. On the one hand, it is quite certain that a classification of the Asteroidea which depends on the number of the rays would exhibit a very incomplete account of the systematic relations of the members of the class ; but, on the other hand, it is just as true that no better name was ever applied to the winged Insecta than that of Hexapoda, or to the higher Vertebrata than that of pentadacfyle ; and it is just as clear that the division of modern Ungulates into two groups, one perissodactyle and the other artiodactyle, could only have been suggested by a naturalist capable of seeing a great general truth through a not always constant similarity in detail. W e now have to weigh these two opposing arguments in applying to the Echinidse (of earlier writers and of Loven) the mode of classification suggested and worked out by Desor1, by which we get the two groups of the Oligopori and Polypori. The test to be applied shall be twofold. First, let us see how it works in the hands of so skilful a naturalist as Prof. Alex. Agassiz. His division of the Echinometradse is defined (as we already! know) as, inter alia, always having more than three pairs of -pores to each arc. But, as a matter of fact, he includes under the Echinometradse the two genera Parasalenia and Echinostrephus. Of the former he says " this genus seems to be an Oligopore among the Echinometradse, having but three pairs of pores in each arc." In speaking of Echinostrephus the generic definition includes no reference to the number of pairs of pores in an arc ; but in speaking of E. molare, the only species of the genus, he says " there are from three to four pairs of pores in each arc, the majority having but three pairs." The other consideration arises from a study of the facts as exhibited in the tests of various species. If in any of these some of the arcs can be shown to possess only three pairs of pores, it seems to m e that such a fact alone would disqualify numerical relations from forming the criteria of generic, or even higher, delimitations. Turning again to the guide we have already followed, we find this sentence : - " Le quatrieme arc, ici muni de quatre pores, n'en a que trois chez quelques individus du Toxopneustes drobachiensis, c'est-a-dire que la plaque composee 3 ne possede qu'une seule plaque primaire me'diane. II y a done quelque variabilite."2 So, again, Dr. Liitken finds in the rare Echinometra oBlonga that, towards either pole of the corona, there are but two or three pairs of pores in each arc3. 1 Synopsis des Ech. fossiles. a J. c. p. 25. to« F?' flg- 1 0 ol t h e first Plate iu his ' Bidrng til Kundskab om Echinoderme,' lrt)4. |