File:American malacological bulletin (1988) (17533638674).jpg

Title: American malacological bulletin
Identifier: americanmal6719881990amer (find matches)
Year: 1983 (1980s)
Authors: American Malacological Union
Subjects: Mollusks; Mollusks
Publisher: (Hattiesburg, Miss. ?) : (American Malacological Union)
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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RUSSELL-HUNTER: GILLS OF CHITONS 73
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Fig. 3. Stereogram of part of a chiton ctenidium. Water is moved dorsally (and pedally) by bands of lateral cilia (1c). On the inhalant (INH) side, the ctenidial leaflet edges bear front cilia (fc) and the ctenidial axis contains the efferent branchial vessel (ebv). On the exhalant (EXH) side, the leaflet edges bear abfrontal cilia (ac) and the gill axis contains afferent branchial vessel (abv). The opposed free tips of the leaflets bear specialized cilia forming the Velcro-like ciliary junctions (x), probably representing modified frontal cilia. the continuity of the ctenidial curtain is maintained even with air-bubbles in the anterior third of the groove on both inhalant and exhalant sides of the gill row. Yonge (1939) also noted that the exhalant opening across the girdle was less variable in position, being always at the posterior end. At first sight this seems true in living Chaetopleura, and the anus in the midline is always swept by a strong exhalant current. However, while the arching of the girdle to form an exhalant opening always occurs close to the anus, its size (with water velocity inversely related) and its direction (to left or to right of the midline) do vary. As such changes occur, accommodation of the ctenidial curtain to pressure shifts involves it becoming less convex (more flattened towards the foot, decreasing the exhalant cavity volume) or more convex (decreasing the in- halant fraction of the pallial cavity). Changes resulting from shifts in size or direction of the exhalant opening can be par- ticularly obvious in a chiton crawling over a curved or irregular surface. Once again, the simplest set-up used to view a chiton through a flat glass surface can be deceptive. Working with both living specimens and models of mopaliid chitons, R. S. Cox and his colleagues have applied water flow visualization techniques in flow tanks and have noted muscular contractions of the pallial groove walls (Douglas J. Eernisse, pers. comm.). They have had only equivocal evidence of pallial shape producing augmentation of flow (such as ramming or Bernoulli effects), but my obser- vations suggest that the chiton's ability to modify the exhalant (downstream) pressure by changes in the effective diameter of its exhalant girdle opening could have some significance in shifting the fluid dynamics of the pallial system. Despite this, basic water propulsion and consequent differential pressures in the pallial compartments must all result from the activity of the lateral cilia on the ctenidial leaflets. It is noteworthy that, even in adult chitons, there are always some bands of ciliated epithelia on the walls of the pallial groove which beat in a posterior direction (particularly on the inside of the girdle). Such ciliation is obvious in young (30-day) postlarval chitons, where it exists before the first ctenidial buds and creates analogous water currents (Russell Hunter and Brown, 1965). However, in adults these cilia seem to propel superficial strings of mucus rather than the ambient water. Despite the adjustments of walls and openings, the dynamic continuity of the ctenidial curtain is maintained as the living chiton crawls along. The direction of the gill axes, with their obvious efferent branchial vessels (Fig. 2), can be seen to be altered but adjacent axes always stay more or less parallel. Groups of six to eight (or occasionally more) gills move together, with their gill-tips lagging behind the foot as the chiton crawls forward, or making a fast recovery so that the gill-tips are seen to be moving forward relative to the edge of the foot. Similarly, groups of ctenidia acting together can move their tips toward and away from the pedal edge. This must involve neural coordination in, for example, the simultaneous contraction of the afferent muscle strands in

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