Comments on the morphology of the pre-oral spine in Argulus (Crustacea: Branchiura).

A histological study of the pre-oral spine and associated structures in Argulus japonicus reveals that two exceptionally long and slender muscles casuse retraction of the spine into the spinal sheath. Originating from the body wall mid-dorsally the two muscles proceed through the nerve ring and the blood-filled lumen of the spinal sheath before being inserted on a bolster of tissue constituting the base of the spine. Movement of the spinal sheath is affected by two muscles originating from a common apodeme in the dorsal body wall, just posterior to the nauplius eye, and inserted on the lateral walls of the base of the spinal sheath. Two ducts, each leading from a glandular complex in the lateral parts of the carapace, proceed rostrally through the haemocoelic spaces. After passing the tritocerebrum laterally they accompany the retractor muscles of the spine on their course to the base of the spine. Passing through the bolster of tissue at the base of the spine the two ducts run all the way up to the tip of the spine where they open separately. The glandula praeboscoidalis is not associated with the pre-oral spine since its duct runs posteriorly in the wall of the proboscis. The labial spines are associated with giant glandular cells located beneath the optic tracts. © 1992 Wiley-Liss, Inc.