| Abstract: | The Japanese pygmy squid, Idiosepius paradoxus (Ortman, 1888), is one of the smallest cephalopods in the world. Their fully described developmental stages and the ready availability of eggs also make pygmy squids suitable material for cephalopod developmental studies. However, their phylogenetic position among the Decabrachia is unclear. We investigated the vascular anatomy of the Japanese pygmy squid using serial sections of the adult. Their venous system is different from that of teuthoids with respect to the presence of the vena cava sinistra. The branching pattern of the vena cava is similar to that of the sepiolids in having two pathways, but different in that it lacks a connection above the hind gut. The Idiosepius heart is also similar to that of sepiids in having the genital artery located dorsally. These features of Idiosepius agree with molecular phylogenies suggesting they are related to the sepiolids. We discuss the venous branching asymmetry with reference to its phylogenetic and functional significance. |
