. Bulletin of the Natural History Museum Zoology. outline. The anterior is approximately twice the size of the posterior and crescent-shaped. At the mantle margin ventral to the anterior adductor muscle there is a well-developed anterior sense organ. This is derived from the middle sensory fold which is greatly extended to form two flaps. The epithe- lial cells of the outer flap are ciliated. Internal to this the epithelium is glandular (Fig. 48c). Underlying this epithelium is a thick layer of connective tissue containing the pallial nerve. The gills lie parallel to the postero-dorsal shell m
. Bulletin of the Natural History Museum Zoology. outline. The anterior is approximately twice the size of the posterior and crescent-shaped. At the mantle margin ventral to the anterior adductor muscle there is a well-developed anterior sense organ. This is derived from the middle sensory fold which is greatly extended to form two flaps. The epithe- lial cells of the outer flap are ciliated. Internal to this the epithelium is glandular (Fig. 48c). Underlying this epithelium is a thick layer of connective tissue containing the pallial nerve. The gills lie parallel to the postero-dorsal shell margin. They are well-developed with up to 22 alternating gill plates (the number relates to the size of the individual). The labial palps are moderately large and, also depending on the size of the animal, have up to 24 ridges on their inner face. The palp proboscides are relatively slender. The palps are wedge-shaped and extend 1/4-1/2 way across the body. The mouth is set posterior to the anterior adductor muscle. The foot is well-developed, and of typical nuculanid form. The divided sole is elongate with papillate edges. At the tips of the papillae, lying between muscle fibres are glandular cells with ducts to the surface of the foot. Secretions from these may be used to lubricate the movement of the foot through the sediment. A large well-developed byssal gland is present in the heel of the foot (Fig. 49a). The byssal gland is spherical, composed of large, hyaline cells, surrounded by muscle fibres. It opens medially at the posterior limit of the divided sole and several glandular epithelial cells surround the opening. A very small medial papilla lies posterior to the opening at the heel of the foot. Histochemical tests have been carried out to identify type secretion from the gland. Tests did not confirm any protein component but this could have been masked by other muco-substances. The main compo- nent at the centre of the byssal gland appears to be a keratin-sulphate (PA
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