Observations on the Function and Physiological Properties of a Fast Paramyosin Muscle—The Notochord Of Amphioxus (Branchiostoma Lanceolatum)

ABSTRACT Measurements of tension and pressure from the isolated notochord during electrical stimulation indicate that the notochord of Amphioxus is a muscular organ. Muscular contraction increases the stiffness of the notochord and the pressure increase suggests that it behaves as a hydrostatic skeleton. The mechanical responses of the muscles accord fairly well with the known characteristics of other paramyosin muscles, although they are rather faster than the paramyosin muscles of most molluscs. The muscular response appears to make its greatest contribution to notochordal stiffness at the small angles of bending associated with fast swimming. Notochordal activation is preceded by a giant fibre potential. The collagen of the notochordal sheath appears to be rather more extensible than vertebrate collagen, but there is a material, probably in the elastica interna, with similar properties to elastin. The characteristics of the lancelet notochord emphasize rather than detract from the importance of its phylogenetic position.