This exper~mental study examines the importance of active and passive processes during the settlement of the barnacle Balanus sp. and the hydrozoan Tubulana crocea larvae. Three hypotheses were tested: (1) larvae of Balanus sp. and 7 crocea exhibit settlement preferences with respect to scales of topographic heterogeneity (0, 1, 10 and 100 mm V-shaped grooves); (2) these preferences are modified by substratum complexity (combinations of scales of topographic heterogeneity); and (3) these preferences are mediated by larval behaviour directly on or near the substratum (benthic behaviour: after initial contact with the substratum). Artificial PVC panels of different scales of heterogeneity and complexity were submerged 30 cm below the surface during autumn 1992, a t Beaufort, North Carolina, USA. A companson between the final attachment sites of larvae with their initial contact sites on complex hard substrata was used to determinc the role of larval exploration following initial contact. Results indicated that, for Balanus sp. and T crocea larvae, all 3 hypotheses were confirmed. Evidence of actlve behaviour was observed at the spatial scales considered In t h~s study (entire panels and types of surfaces within panels), suggesting that active selection occurs at smaller and smaller scales. Balanus sp. larvae were rllure abundant on pancls which d~d not include the 1 mm scale of heterogeneity while, at the level of types of surfaces within each panel, they settled preferentially in grooves of 1 and 10 mm. T h~s suggests that different factors affect the competent 1a1-vae at d~fferent spatial scales. For T. crocea, the larvae selected the most complex type of panels and generally settled on the most exposed surfaces. Differences between initial larval contact sites and flnal attachment sltes confirmed the benthic behaviour hypothesis.

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par Michel LEMIRE Dans une premiere partie est envisaged la structure histologique des diverses regions de « estomac > sensu lato du Lamantin du S£n6gal. Cette eiude permet de dofinir une nouvelle zonation des territoires fonctionnels de la portion antarieure du tube digestif des Sire*niens.L'analyse morphologique est accompagnoe d'une discussion portant sur Phisto-physiologie de la digestion chez les Sir£niens. Ces considerations laissent entrevoir certaines relations entre l'habitat et le regime alimentaire d'une part et les processus secretaires et microbiologiques d'autre part. Pour etayer ses conceptions, Pauteur fait appel ä des donnoes qualitatives et quantitatives acquises cbez des c Ve^tariens > terrestres tels que les Ruminants. Aux singularity anatomiques marquantes du dispositif c gastrique » des Siraniens ne rlpondent pas des bouleversements pbysiologiques profonds, mais plutot des c am£nagements » du processus fondamental de digestion des Mammiferes. Ces arrangements, tant histo-morphologiques qu'histo-physiologiques, tomoignent, semble-t-il, d'une double adaptation : a l'habitat aquatique et au regime alimentaire vlgeiarien des Sironiens.(1) Nous donnons ci-dessous, ä titre indicatif, quelques mensurations effectuoes sur l'individu adulte en notre possession, l'estomac £tant au repos : premiere poche : longueur totale = 15 cm, plus grand diametre = 6 cm ; seconde pocbe : longueur totale -10 cm, plus grand diametre = 3 cm; appendice « cardiaque > : longueur -9 cm, diamotre = 4 cm; caecum € pylorique » : longueur = 9 cm, diametre = 2 cm.Brought to you by | University of California Authenticated Download Date | 6/7/15 8:26 AM

The optic nerve in two species of subterranean mole-rats (Spalacidae) has been examined at the ultrastructural level. The axial length of the eye and the diameter of the optic nerve are 1.9 mm and 52.5 μm in Spalax leucodon, and 0.7 mm and 80.8 μm in Spalax ehrenbergi, respectively. An anti-glial fibrillary acidic protein postembedding procedure was used to distinguish glial cell processes from axons. In both species, the optic nerve is composed exclusively of unmyelinated axons and a spatial distribution gradient according to the size or the density of fibers is lacking. The optic nerve of S. leucodon contains 1790 fibers ranging in diameter from 0.07–2.30 μm (mean = 0.57 μm), whereas in S. ehrenbergi, only 928 fibers, with diameters of 0.04–1.77 μm (mean = 0.53 μm) are observed. In S. ehrenbergi, a higher proportion of glial tissue is present and the fascicular organization of optic fibers is less obvious. Distribution gradients according to size frequency or density of fibers in the optic nerve are absent in both species. Comparison with other mammals suggests that although ocular regression in microphthalmic species is correlated with a significant decrease in the total number of optic fibers and the relative proportion of myelinated fibers, no difference in the absolute size range of unmyelinated axons is observed. The total absence of myelinated fibers in Spalax may be related to the subcutaneous location of the eyes. The unique presence of unmyelinated fibers in the optic nerve is discussed in relation to the possible conservation of a single class of W-like ganglion cells in the retina, in relation to photoperiodic perception.

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