Sensory innervation of the tentacles of the polychaete, Sabella pavonina

Summary

Following observation of conical groups of stiff, but motile cilia on the tentacles of the branchial crown of Sabella pavonina, these were examined with the electron microscope. The bundles consist of about 40 unenclosed “standard” cilia supported by one or two primary sense cells with centrally directed axons of 0.1–0.2 μ diameter. Axons in the distal portions of the branchial crown occur in small bundles surrounded by a basement membrane. More centrally, glial elements appear and the nerves are surrounded by a collagenous sheath. The branchial nerve trunk shows similarities in organisation to other previously investigated annelid central nervous tissue in that the whole nerve is surrounded by a fibrous sheath central to which there is a layer of glial cells with processes penetrating a central neuropile. The 0.1–0.2 μ axons commonly occur in glial-enveloped groups of < 40 whilst other axons of larger and mixed diameter are found together.

Each tentacle has two branchial nerves on the oral side, and each nerve gives rise to two small 75-axon branches running to each pinnule. The branchial nerves fuse to form the branchial nerve trunk running to the supra-oesophageal ganglia.

Sections of the branchial nerves of the branchial crown at progressively more central levels show that the branchial nerve trunk contains enough axons of 0.1–0.2 μ diameter to account for all the sensory cells on the tentacles. This is taken as evidence for the sensory cells having axons terminating within the central nervous system and that there is no peripheral confluence or fusion of these afferent axons.