. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. APLACOPHORA: PROGENETIC COELOMATES 69 (1985) and Salvini-Plawen (1972) suggest that the sub- rectal commissure is an apomorphy. Whichever interpre- tation is correct, one can say that monoplacophoran and aplacophoran-polyplacophoran nervous systems are each apomorphic to some unknown ancestral state, and the suprarectal ganglion or commissure of the Aplacophora- Polyplacophora serves to relate them phylogenetically and set them apart from the Monoplacophora. Spicule formation Spicule formation in aplacophorans and polyplacop
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. APLACOPHORA: PROGENETIC COELOMATES 69 (1985) and Salvini-Plawen (1972) suggest that the sub- rectal commissure is an apomorphy. Whichever interpre- tation is correct, one can say that monoplacophoran and aplacophoran-polyplacophoran nervous systems are each apomorphic to some unknown ancestral state, and the suprarectal ganglion or commissure of the Aplacophora- Polyplacophora serves to relate them phylogenetically and set them apart from the Monoplacophora. Spicule formation Spicule formation in aplacophorans and polyplacoph- orans has most recently been investigated by Haas (1981) (Fig. 10). Spicules in both taxa are aragonite and formed extracellularly within an invagination of a single basal cell, which secretes calcium carbonate within a crystalli- zation chamber sealed by neighboring cells (Scheltema a al., in press, fig. 6D). In chitons, megaspines are formed from a proliferation of the basal cell and do not occur in aplacophorans. Spicules of the Aculifera are usually considered to be a plesiomorphic state of calcium carbonate formation within Mollusca, since both spines and shell occur in chi- tons and only spines occur in Aplacophora, both being "primitive" groups in the general sense. However, Mono- placophora, likewise considered primitive, have no spines. The dorsal, calcium-carbonate-secreting epidermis of Mollusca, in combination with a ventral locomotary sur- face, is probably an apomorphy. However, the shell-bear- ing Brachiopoda are rooted with the Mollusca-Annelida group by RNA sequencing (Field el 1988). and some boring Sipuncula have calcium carbonate deposits at the dorsal anterior end of the trunk (Rice, 1969). Further comparative work needs to be done to compare calcium carbonate secretion among the Eutrochozoa before ho- mology can be assumed. It cannot be concluded from outgroup comparison that spicules and shell are homologous structures (and the ar- gumen
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
