File:American malacological bulletin (1988) (17970002259).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

View Book Page: Book Viewer
About This Book: Catalog Entry
View All Images: All Images From Book
Click here to view book online to see this illustration in context in a browseable online version of this book.

Text Appearing Before Image:
72 AMER. MALAC. BULL. 6(1) (1988)
Text Appearing After Image:
Fig. 2. Diagrammatic view of four ctenidia in Chaetopleura from the ventral inhalant side (INH). The axes show the efferent branchial vessels (ebv), and there are frontal cilia on the facing edges of the leaflets. Note that the tips of leaflets are opposed one to one, or one to two (INH. inhalant current; gi, girdle; ft, foot; pg, pallial groove). toward the axial ciliary tract (Fig. 3) and thence toward the gill tip. In a healthy chiton, accumulation of this sort at the tip provokes a reflex action sequence. The reflex is not gravi- ty dependent and can be observed in chitons in all postural relations to the horizontal. The same reflex takes place if foreign material is loaded on the abfrontal (exhalant) face of the gill. The patterned cleansing reflex occurs in three sequen- tial phases. First, for two to three seconds, more blood is pushed in while the gill expands. (It is difficult to measure this, but the overall volume increase at this phase is usually be- tween 20% and 50%). Secondly, the muscle strands under the afferent branchial vessel contract relatively slowly, taking between 2 and 5 seconds. Thirdly, the muscle under the ef- ferent branchial vessel contracts relatively rapidly, taking be- tween 0.1 and 0.2 seconds, and flicks the tip toward the foot and substratum-surface while simultaneously shortening the gill. (Only at this stage is the contained blood volume reduced again.) In most cases a mucous-bound pellet of natural sedi- ment, or foreign particles, leaves the gill surface and remains on the cilia of the pedal edge or on the substratum. It should be noted that there is never any question of ciliary junctions being formed (even temporarily) between the gill tip cilia and the pedal cilia. Despite the subtle differences in ctenidial proportions noted above, this reflex action of the individual chiton gill (ac- ting, it seems, in a neuromuscular sense as a peripheral reflex, or almost as an independent effector system) involves a patterned sequence exactly following that observed in the aspidobranch gill of Acmaea. Parenthetically, it is worth noting one somewhat special case observed in living chitons, con- cerning the last large gill in Chaetopleura. On several occa- sions it has been observed to "flick" material right out of the pallial groove, with its tip passing under the girdle in a tem- porary (and asymmetric) lifting more like the usual local arch- ing of the girdle for typical temporary inhalant openings. Of course, this can only occur because of the distinctly different siting of that last large gill, with its tip directed posteriorly and girdlewards instead of toward the midline and foot. In this respect as others, conditions in the lepidopleurid chitons must be quite different, but we lack observations of living gill movements. In typically near-holobranch chitons like Chaetopleura and Lepidochitona, groups of three or more ctenidia can flick together. This leads to the second group of new observations. THE COORDINATED CTENIDIAL CURTAIN Even the casual observer of the underside of a living chiton can see (Fig. 2) the functional organization of each row of chiton gills into a pallial curtain dividing the mantle groove along most of its length into inhalant and exhalant chambers. This is functionally dependent upon the occurrence of Velcro- like ciliary fastenings on the leaflet tips of chiton gills. Unlike the ciliary junctions in mytilid and other "filibranch" bivalves which are modified lateral cilia linking adjacent filaments on the same ctenidium, these ciliary junctions in chitons link leaflets on adjacent gills and probably represent modified fron- tal cilia. If the filibranch gills typical of mussels, scallops or oysters are disturbed mechanically, the ctenidial filaments become tangled and the coordinated filtering and sorting func- tions are temporarily lost. Given otherwise healthy conditions and a little time (usually only a few minutes), the filaments will "crawl" by ciliary action over each other until the ap- propriate ciliary junctions are reconnected and the seeming- ly continuous corrugated lamella re-established as a porous water-propelling and filtering surface. Similar processes oc- cur if the ctenidial curtain is mechanically disturbed in a healthy chiton. Individual gills can carry out slower flicks across the pallial groove, but the main re-establishment of the curtain involves the ctenidial tips being "walked" (largely by ciliary action) along the side and edge of the foot, and over each other until an orderly row is again set up. With re- establishment of the row, the ciliary junctions reconnect the tip of one posterior leaflet either to one or to two anterior leaflets on the gill behind it. In healthy chitons, the way in which each ctenidial row moves as a single dynamic curtain is impressive. It bulges and flattens to accommodate changes in the hydraulics of the pallial groove resulting from shifts in the inhalant (and less frequently the exhalant) openings across the girdle as the chiton crawls along. The early observation of Yonge (1939) that inhalant openings can be formed by local lifting of the girdle at almost any point along the anterior part of the chiton is clearly confirmed. Yonge's conjecture, that the capacity for creating inhalant openings back along the sides of the body is valuable when the anterior end is out of the water, can be supported by the observation that, in Chaetopleura at least,

Note About Images