- Polyplacophora Each of the eight valves of the shell is made up of two distinct calcareous layers: (a) an outer or upper called the tegmentum, which is visible externally; (b) a deeper layer called articula FIG.
The articulamentum projects laterally beyond and beneath the tegmentum into the substance of the mantle.
The tegmentum is formed by the fold of mantle covering the From Lankester, Treatise on Zoology.
The tegmentum is much reduced in Acanthochiton, and absent in the adult Cryptochiton.
The tegmentum is pierced by numerous vertical ramified canals which contain epithelial papillae of the epidermis.
Nk, ng, valves arise as transverse thickenings of the dorsal cuticle behind the ciliated ring, the tegmentum being the first part formed.
- Tegmentum coextensive with articulamentum, or the latter projecting in smooth unslit plates.
No shell-eyes, slits in the valves 1-7 corresponding with the ribs of the tegmentum.
Valves immersed in the girdle, with small tegmentum.
- No shell-eyes: sutural laminae separated; slits in the valves 1 -7 do not correspond with the ribs of the tegmentum.