THE MYOTOME FORMULA OF THE LANCELET EPIGONICHTHYS LUCAYANUM (ACRANIA)- CAN VARIATIONS BE RELATED TO LARVAL DISPERSION PATTERNS

Variation in the myotome formula of the acraniate Epigonichthys lucaya num (Andrews) is discussed in relation to that of its putative larval stage Amphioxides pelagicus (Gunther). In certain adult populations, t wo main forms can be recognized: typically, one form has 58-61 myotome s, of which 38-40 are pre-atrioporal, and the other form has 65-69 myo tomes, of which 43-45 are pre-atrioporal. These two adult forms are th ought to be derived from different larval stages, the former from an a mphioxus larva-like stage that has metamorphosed after a short plankto nic phase and the latter from the amphioxides stage which has undergon e a long, more widely dispersed, planktonic existence. The adaptations for a prolonged planktonic life developed in the amphioxides, especia lly gill-slit development, appear to be reflected to some extent in th e meristic characters of the resulting adult.