BIODIVERSITY AT THE CELLULAR AND SUPRA-CELLULAR LEVELS OF THE MUSCULOSKELETAL COMPLEX OF CHAETOGNATHS

Chaetognath trunk and tail basic structural patterns show the importan ce of their hydroskeleton antagonistic of a powerful parietal longitud inal locomotory muscule. Three types of trunk muscles are sometimes fo und : the primary and the secondary muscle, a transversal muscle is pr esent in archaic, bentho-planktonic and benthic animals, it is reduced in one planktonic genus. In the more or less archaic forms, the prima ry muscle shows 4 distinct masses, where the two kinds of A and B fibe rs are gathered in units; in the most of the species, the A fiber unit s are arranged helicoidaly. In the trunk, two A fibers units alternate with one B fibers unit; in the tail the alternation is 1A/1B. The rat io between the number of A and B fibers units varies according to the species;their volume ratio shows that the archaic species have B fiber s which are poorly developed. Some benthic animals, as Spadella, lack B fibers. At present, the secondary muscle is only found in the phylum . Some correlations between the broad lines of phylogeny and ecology c an be proposed.