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Show 1881.] PROF. F. J. BELL ON THE ECHINOMETRIDsE. 413 downwards to which the test is subjected, and to the widening-out of the composite plates during the process of growth. I have dwelt at this length on the results of Prof. Loven's labours, not merely for the purpose of directing again attention to them', but with the more especial aim of showing that it is only on a misconception of the history that one can speak of secondary plates as different from those first formed, or of such being added on to the sides of a primary plate. But the origin of such a misconception is not far to seek ; it must surely be due to a study of the arrangement of the pores of the adult, and be comparable to the formulas of Milne-Edwards and Haime as applied to the structure of the coral-septa; while M . Loven's work will stand no less on an equality with the elegant and instructive researches of Lacaze- Duthiers2. Armed with this knowledge we come now to a consideration of the value of the characters of the arcs of pores. It has been proposed to distinguish the family of the Echinometridse from the Echinidse proper on the ground that the former have always more than three pairs of pores to each arc, " while in the Echinidse the arcs are always composed only of three pairs." " This division, although it appears a numerical one, is. yet one of great physiological importance, as the mode of growth of the poriferous zone in these two families is totally unlike "3. I am inclined to think that the accomplished author is here using the term physiological in some other sense than that to which its etymology and the current usage of qualified persons justly entitles it; he is too experienced a zoologist to attempt to make the functions of organs do the work of morphological and embryological data. However, the mode of growth of the pores is as much matter for morphologists as for physiologists ; and the only question which really arises here is, as to the real character of this total unlikeness. If such exists, it may or may not he of value. But, first of all, does it exist ? Prof. Loven says4:-" Les chiffres par lesquels la disposition des pores est designee chez cette espece, les 2, 3, 3, 4, etc. de la serie I a . . V B, et les 2, 2, 3, 4, etc. de la serie I B . . V a, se retrouvent non seulement dans les especes voisines, le Toxopneustes Brevispinosus (Risso) et le T. lividus (Lamk.), mais encore dans le Loxechinus alBus (Mol.), YEchinus esculentus, L., le Lytechinus variegatus (Lamk.), le Tripneustes ventricosus (Lamk.), la Boletia heteropora, Desor, YAmBlypneustes ovum (Lamk.), le Temnopleurus toreumaticus (Leske), 1'Echinothrix turcarum, Peters, VEchinocidaris punctulata (Lamk.), en un mot chez tous les Echinides. Les Echinometra n'y font pas exception." So far, then, as the formation of the two separate families Echinometridse and Echinidse is based on the difference in the mode of 1 A short account is to be found in Prof. Huxley's ' Anatomy of Invertebrated Animals'(1877), p. 568. 2 Archiv de Zool. Exp. vol. i. 3 Eev. of the Echini, p. 423. * T. c. p. 26. |