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Show 1881.] PROF. F. J. BELL ON THE ECHINOMETRIDsE. 415 And, dealing with the restricted genus Echinometra, I have myself been able to make somewhat similar observations in the case of E. oBlonga, E. suBangularis, and E. lucunter. If we put into a tabular form the numbers of pairs of pores in an arc, we find the average adult arrangement to present very considerable variation, thus:- Echinometra lucunter has 5 or 4 pairs of pores. E. oBlonga „ 5 „ ,, E. viridis „ 5 „ ,, E. suBangularis ,, 6 or 5 „ ,, E. macrostoma „ 8, 7, or even 3 pairs of pores. E. vanbrunti ,, 9, 8, or 7 pairs of pores. Having already insisted on the fact that only three pairs of pores are to be detected in the youngest plates, or some of the plates, on the test of certain so-called polyporous species, we will insist as much as possible on the "polypority " of these forms by detailing the arrangements which are found in the best-developed part of the adult test1. (1) Parasalenia gratiosa .... 3 pairs of pores. (2) Echinostrephus molare . . 3 (or 4) ,, ,, (3) Echinometra lucunter .... 4 or 5 (4) E. oblonga 4 or 5 (5) E. suBangularis 5 or 6 (6) Spha?rechinus granulans .. 4, 5, or 6 (7) E. macrostoma 7 or 8 (8) E. vanbrunti 7, 8, or 9 (9) Strongylocentrotus bullatus 7 or 8 (10) S. franciscanus 9 (11) 8. alBus 10 (J 2) GoloBocentrotus atratus .. 12(ca.) (13) Heterocentrotus trigonarius 15 (ca.) „ „ We come, then, to the following results:- (1) There is a series of forms which exhibits a gradual increase in the number of primary plates which go to form a secondary plate; but (2) This series always retains indications of secondary plates in which only one primary plate has ceased to grow with the rest. When we come to take a general survey of the characters exhibited by the pore-plates, we find the primitive arrangement of the Cidaris passing gradually into the complex secondary plates of Heterocentrotus and ColoBocentrotus, which would appear to be the most highly differentiated of the Desmosticha. 1 Bearing in mind the words of Prof. Gegenbaur (Unters. zur vergl. Anat. d. Wirbelthiere (1864), i. p. 116), "So wichtig es ist fur die Aufstellung von Unterschieden der einzelnen Lebensformen und ihrer Einrichtung, die ausge-bildeten fertigen Zustande der Organisation zu Unterscheidungsobjecten zu nehmen ;" and Flower, Introductory Lecture (1870), p. 37- >> 5) |