File:American malacological bulletin (1988) (17533596994).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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although they are better regarded as symplesiomorphic traits, shared primitive states that do not necessarily show close evolutionary relationships. Beedham and Trueman (1968) found similarities in the histochemistry of aplacophoran and chiton integumental cuti- cle and concluded that "the cuticle of the Aplacophora is ten- tatively equated with an early mucoid stage in the evolution of the molluscan shell... (The cuticle of Acanthochiton) has in addition a discrete inner cuticular layer which may act as a semi-conducting membrane in the deposition of calcareous plates" (p. 443). The papillae of Aplacophora and Poly- placophora are probably homologous (F. P. Fischer, pers. comm.); the papillae and aesthetes of Polyplacophora are likewise homologous (Fischer et al., 1980; Fischer, 1988). The process of calcareous spicule formation, most recently investigated by Haas (1981), is alike in aplacophorans and chitons (Fig. 2). In both taxa, a spine is secreted extra- cellularly within an invagination of a single cell. A basal cell secretes calcium carbonate, and as the spicule grows beyond this cell, a crystallization chamber is sealed off by a collar of neighboring cells. The megaspines in chitons, which do not occur in Aplacophora, are formed by a proliferation of the original single basal cell. The attempt to find further similarities in calcium car- bonate deposition that would link the Aplacophora and Polyplacophora by examining embryogenesis has led to less conclusive comparisons. Larval development in the two
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Fig. 2. Spicule formation in Aplacophora and Polyplacophora. A. Primitive Neomeniomorpha. B. Lepidochitona cinerea (Linnaeus). An organic pellicle has not been demonstrated around spicules of the Aplacophora. (After Haas, 1981.) (b, basal cell; n, neighboring cell; p, organic pelli- cle; s, spicule). Scale bars = 1 ^m.

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