THE FINE MORPHOLOGY OF THE SHELL SAC IN THE SQUID GENUS LOLIGO (MOLLUSCA, CEPHALOPODA) - FEATURES OF A MODIFIED CONCHIFERAN PROGRAM

The supposedly ancestral chambered shell design is conserved in only a few extant cephalopod groups (in the ectocochlean Nautiloidea and in two families of the endocochlean Coleoidea: Sepiidae, Spirulidae); the majority of living cephalopods have purely organic shells or shell re lies produced by secretory epithelia of the closed shell sac which lie s in the dorsal part of the muscular mantle. The ultrastructure of the se epithelia is described here from a loliginid squid, providing the b asis for discussing the extent of phyletic conservation of the ''conch iferan program'' in a highly modified morphogenesis. The squid shell d evelopment omits both chamber formation (the ancestral cephalopod prog ram related to a calcified shell) and shell calcification (the ancestr al conchiferan program), but shows features that are recognizable as c onserved elements of the conchiferan mode of shell formation. The grea tly ''simplified'' squid shell development is interpreted as part of a n adaptive process by which full integration of the shell into a highl y flexible muscular mantle was achieved.