File:American malacological bulletin (1988) (17970000079).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 71
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Fig. 1. Diagrams of the mantle groove in a chiton (based on Lepidochitona) showing (A) ventral aspect of anterior part of groove, and (B) cross- section of the groove at a central ctenidium. Note that inhalant part of the groove (INH) is girdle-ventral and exhalant part (EXH) is pedal-dorsal (gi, girdle; ft, foot; pg, pallial groove). each ctenidial leaflet. On the inhalant (INH) side the edges of the leaflets bear shorter frontal cilia (that is, on the facing edges of Fig. 2), and on the exhalant (EXH) side the leaflet edges bear abfrontal cilia. Both frontal and abfrontal cilia have a cleansing (particle-moving) function rather than water pro- pulsion, and transport particles around the leaflet edges toward the axis. The free tips of each leaflet bear specialized longer, less motile cilia that entangle in a Velcro-like fasten- ing (x on Fig. 3) with the corresponding cilia on the leaflet tips of the adjacent ctenidium. From their position, and development in ctenidial buds, these ciliary junctions linking adjacent gills probably represent modified frontal cilia. The assemblage of microstructures and their functions shown by the chiton gill are thus essentially similar to those found in the primitive "aspidobranch" plume gill of the Ar- chaeogastropoda. If an individual chiton gill is specifically compared with the single plume gill in the limpet, Acmaea testudinalis (Mu'ller), the only significant difference involves the Velcro-like ciliary junctions on the chiton leaflet tips. There are obviously minor differences of microanatomy such as the outline proportions of the leaflets, and the distribution of lateral cilia on the leaflet faces, but these seem trivial in comparison with the broader concert of structures and functions. The gill axes with alternating leaflets are essentially identical in ar- rangement, as are the dorsal afferent branchial vessel and the ventral efferent vessel carrying oxgenated blood back to the heart. The lateral, frontal and abfrontal cilia are arranged in the same way and, in both, the lateral cilia produce a flow of water through the gill (and through the mantle cavity) in the opposite direction to the blood flow. Chiton gills are true ctenidia, structurally and functionally homologous with those of other molluscs. The rows of chiton gills are clearly not neomorphic structures, secondary respiratory organs as in some marine limpets like Patella, or in various groups of freshwater pulmonate snails (Russell-Hunter, 1978; McMahon, 1983), but have to be regarded as rows of multiplied ctenidia. CTENIDIAL CLEANSING REFLEX Surprisingly little attention has been paid to the muscular movements of primitive molluscan ctenidia. A relatively new set of observations on chiton gills concerns the fact that each ctenidium can move in a patterned cleansing reflex. To anticipate a little, the sequence of movements in the individual chiton ctenidium seems to be exactly similar to that in the cleansing "flick" of the single plume gill in forms like Acmaea. In the axis of the chiton ctenidium, longitudinal mus- cle fibers lie around and below the two major blood vessels. When both sets of muscle strands contract together, the gill is shortened and pulled toward its base, with a consequent decrease in the gill's contained blood volume. Gill retraction of this sort can be accomplished in 0.2 to 0.8 seconds. Re- extension of the gill is always slower (several seconds) with blood being passed in hydraulically by action of distant an- tagonists. If the muscle under the afferent branchial vessel alone contracts (stretching the muscle on the efferent side) then the gill curls up into the pallial groove, the ctenidial tip moving away from the foot (Figs. 1, 2). In the opposite case, if the muscle under the efferent branchial vessel contracts the whole gill is straightened and its tip could hit the foot edge or the substratum-surface or both. If the cleansing cilia (frontal) are experimentally load- ed by introducing material (suitably dense but small, like fine grade carborundum) onto the inhalant face of the gill, the foreign particles become mucous-bound and are moved

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