File:American malacological bulletin (1988) (17533601004).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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60 AMER. MALAC. BULL. 6(1) (1988) Deposition of a shell plate anlage takes place within a transverse depression bounded and sealed off by long, overlapping microvilli that lie beneath a gelatinous mucoid substance, certainly not periostracum, and questionably equated with a cuticle (Fig. 4C, D) (Kniprath, 1980; Haas et a/., 1980; Haas, 1981). Not only are the ontogenetic processes of shell forma- tion different in chitons and the Conchifera, but structures of the fully formed shells are also unlike and homologies are difficult to discover. Periostracum in the Conchifera, a struc- ture conservative in manner of its secretion and in composi- e
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i 1 Fig. 4. Larvel shell deposition in (A, B) the gastropod Aolidia papulosa (Linnaeus) and (C, D) the chiton Ischnochiton rissoi (Payaudreau). In A, an organic pellicle (arrows) covers the lumen of the shell field invagination (L); in B, the edge of the pellicle can be seen to be overlain by a cytoplasmic extension (e). Calcium carbonate has not yet been deposited. (Drawn after photographs in Eyster and Morse, 1984: Figs. 1, 2). In C, calcium carbonate of the shell plate (p) has been deposited under the overlapped microvilli (s, "stragulum"); a mucus layer (m) covers the stragulum. In D, microvillar processes (s) have pulled apart and a cuticle (c) with a contrasted outer layer is beginning to form; M is perhaps a mucus cell (C and D after Kniprath, 1980.) Scale bars: A = 10 /*m; B = 0.5 /jm; C; D approx- imately 6 urn. tion (Gregoire, 1972), does not exist in chitons, although Haas (1981) has demonstrated the presence of a thin cuticle, or pro- periostracum, overlying the tegmentum and a properiostracal groove surrounding each shell plate. There is no nacreous layer in chiton shells as found in other mollusks, and the cross- lamellar structure of the shell plates is crystallographically uni- que, with bundles of crystal fibers in the lamellae ordered so that their c-axis "coincides with the bisectrix of these cross- ing fibers" (Haas, 1981: 403) and the "whole complex acts crystallographically as a single crystal" (Haas, 1977: 392). In other molluscan cross-lamellar structures, the angle between crystal fibers is about 110°; in gastropods they lie between 90°-130° (Wilbur and Saleuddin, 1983). Haas (1981) considered the cross-lamellar structure of chitons to be homologous with the nacreous layer of other shelled mollusks and imagined that both arose from an undifferentiated inner layer of the "archiplacophoran" plates. The shell of the Conchifera became univalved he believed by fusion of the shell and shell fields. There is no evidence, however, that the dynamics in- volved in the process of earliest shell deposition through the interplay of shell-field invagination and pellicle in Conchifera could have evolved from the very different process of shell- plate production found in chitons. Thus, recent work on the ontogeny and structure of shell in chitons and Conchifera shows such major differences between them that it can be questioned whether there was a monophyletic origin of molluscan shell, or rather one origin for chitons and a second for the remaining extant and extinct Conchifera. Tubules in the shells of the monoplacophoran Neopilina (Schmidt, 1959), bivalves (e.g. Waller, 1980), and gastropods have sometimes been considered homologous with the aesthete canals of chitons and argued as a support for a monophyletic origin of molluscan shell (e.g. Salvini- Plawen, 1985), but the homology is so far uncertain. When the ontogenetic development of Neopilina becomes known, perhaps a basis will be found for deciding whether molluscan shell has a monophyletic or polyphyletic origin. CHAETODERM ORAL SHIELD AND THE ARCHIMOLLUSK One of the original arguments for dividing the Aplacophora into two classes and, ultimately, into two sub- phyla depends on the hypothesis that mollusks have a turbellariomorph, or flatworm, ancestry. This phylogeny is based on a supposed homology and similarity in mode of locomotion between mollusks and flatworms by means of a "ventral mucociliary gliding surface" (Salvini-Plawen, 1972, 1980: Fig. 5, 1985; see also Trueman, 1976). The molluscan archetype, like the flatworms, is said not to possess a separa- tion of the head from the foot, and the mouth consequently opens through the sole; innervation of the sole is said to be from both the cerebral ganglia and ventral nerve cord. (Stasek (1972) has illustrated but not discussed a head separate from the locomotory sole in the turbellariomorph molluscan precursor) Support for the flatworm-like archimolluscan locomo-

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