THE ESTHETAL CANAL SYSTEM IN POLYPLACOPHORANS (MOLLUSCA) AND A COMPARISON WITH THE SCLERITE CANAL SYSTEM IN SACHITIDS (LOWER TO MIDDLE CAMBRIAN)

Chitons or polyplacophorans are characterized by having eight dorsal shell plates and a surrounding girdle with embedded sclerites. The outer mineralized shell layer is unique among molluscs, being occupied of a complex canal system with cells of secretory and sensory function, termed the esthetes.

The Sachitida He, 1980 (Lower to Middle Cambrian) where forms like Halkieria Poulsen, 1967 and Wiwaxia Walcott, 1911 belong, are common constituents of early Cambrian small shelly fossil assemblages. They sometimes exhibit a canal system within their sclerites. The significance of the canal system and the function of this canal system as well as the systematic position of the sachitids within the metazoans have been debated for some time. New arguments for relating sachitids to the molluscs, especially the chitons have been presented recently. This assignment is opposed to earlier suggestions of relating the sachitids to the brachiopods, annelids or lophotrochozoans in general which are not convincingly supported.

The external morphology of the esthetal canal system has been studied in modern chitons in a number of families; its morphological and ontogenetical constraints are well defined. This is used to deduce whether it is plausible to interpret a similar function for the sachitid sclerite canal system. Acid macerated material of Sinosachites (=Thambetolepis) delicatus from Australia, which is abundant and preserved in great detail, has been used in this study.

There are a number of remarkable similarities between these two canal systems in external morphology and ontogeny, which would further support the relation of Halkieria and the other sachitids to the molluscs. This evidence is used to discuss the early evolution of hard parts and shell pores in basal molluscs.