. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 70 A. H. SCHELTEMA Aculifera as the Sister Taxon of the Conchifera Chitons provide evidence that Aculifera are separate from their sister group, the Conchifera. The evidence is based on shell ontogeny, shell structure, and perhaps mo- lecular data. She/I ontogeny In Conchifera, the shell originates within an ectodermal invagination. the shell-field invagination. which is covered by an organic pellicle (Eyster and Morse, 1984) (Fig. 11). In Aeolidia papillosa, long cytoplasmic processes overlie the pellicle. In chitons, there
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 70 A. H. SCHELTEMA Aculifera as the Sister Taxon of the Conchifera Chitons provide evidence that Aculifera are separate from their sister group, the Conchifera. The evidence is based on shell ontogeny, shell structure, and perhaps mo- lecular data. She/I ontogeny In Conchifera, the shell originates within an ectodermal invagination. the shell-field invagination. which is covered by an organic pellicle (Eyster and Morse, 1984) (Fig. 11). In Aeolidia papillosa, long cytoplasmic processes overlie the pellicle. In chitons, there is no shell field invagination, and shell plate anlagen are deposited within transverse depressions which are sealed, not by a pellicle, but by long, overlapping microvilli that lie beneath a gelatinous mucoid substance (Kniprath, 1980; Haas cl a/., 1980; Haas, 1981; see Scheltema, 1988, for a more complete discussion). Furthermore, in healthy larvae, shell is not deposited as separate granules, as illustrated by Kowalev- sky (1883), but as uninterrupted rods (Kniprath, 1980). This fact conflicts with the hypothesis that chiton shell arose from fused spicules (Salvini-Plawen, 1985. 1990). Shell structure The crystallography of chiton shell has been said to indicate an autapomorphy of chitons by Haas (1976), who found that "The . . c-axis of [the] hypostracum lies in the bisectrix of the crystalline fibers. The whole complex acts crystallographically as a single crystal" (p. 392). If this crystallographic orientation is correct, then no ho- mology exists between polyplacophoran and conchiferan shell. Further differences are a lack of true periostracum in chitons (although Haas [ 1981 ] has demonstrated a thin cuticle overlying the shell plates) and a lack of a nacreous layer (for further discussion see Wingstrand, 1985; and Scheltema, 1988). On the other hand, the shell of the try- blidiacean Monoplacophora does not differ from other primitive conchiferan shells (Lemc
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
