Abstract
Bathyarca pectunculoides (Scacchi, 1834), a small Atlantic species, most noticeably possesses posterior extensions of the inner mantle folds which serve as simple siphons channelling inhalant and exhalant currents, discharging pseudofaeces, and permitting exchange with the water above even if the shell is covered by silt. The ctenidia are large, of the typical arcoid plan and ciliation, with the labial palps small. The ciliary currents of the mantle and visceral mass are powerful. A pallial gland, with a structure similar to that of the primitive bivalve hypobranchial gland, occurs on the posterior region of each mantle lobe and probably consolidates waste material to be expelled into the exhalant stream.
The mantle margin comprises three folds as in the typical bivalve. The suggestion of Waller that the Arcoida possess an arrangement of pallial folds different from all other bivalves is discussed and it is concluded that in essence the same three folds occur, but that in the shallow-water Arcoida a special photosensory sub-fold of the outer fold has evolved, whereas in other bivalves this sensory function is typically an attribute of the middle fold.
Bathyarca is modioliform, with a reduced anterior shell face and anterior adductor muscle. Shell form is typical of nestling species. Probably Bathyarca has evolved from an infaunal endobyssate shallow-water ancestor and the trend towards hetero-myarianism has resulted from, because of functional conservatism in the Arcoida, residence on the surface of the sediment, byssal attachment to sediment particles permitting this.
This rare example of a deep-water suspension feeder (most other deep water bivalves being either deposit feeders or predators), is interesting in revealing how a primitive lineage has evolved mechanisms permitting colonization of soft deposits.
Though normally placed in the Arcidae, B. pectunculoides possesses an arrangement of hinge teeth that more properly ally it with the extinct Palaeozoic Cyrtodontidae or Mesozoic Parallelodontidae. The question is posed: is Bathyarca a living remnant of an otherwise extinct primitive arcacean and pteriomorph stock?
