Japanese pygmy squid, Idiosepius paradoxus Ortman, 1888 is one of the smallest cephalopods in the world. Their fully described developmental stages and availability of eggs also make them suitable material for cephalopod developmental studies. However, their phylogenetic position in decapodiformes is not clear. We investigated the vascular anatomy by use of serial sections of the adult. Their venous system is different from that of teuthoids with respect to presence of vena cava sinistra. The Idiosepius heart is similar to that of sepiids, but not to sepiolids. Subsequently, we found a VEGFR orthologue, which is a major inducer of vascular formation in vertebrates, from the pygmy squid. We demonstrated by in situ hybridization that the squid VEGFR is expressed in cardiac Cephalopod International Advisory Council Symposium (CIAC09) - 61 - field in addition to endothelial cells in the embryos. In situ hybridization with VEGFR provides a useful method for investigating the vascular development. The cells expressing the VEGFR were elongated with filopodia and penetrated into arms, brain, and mantle at the late organogenetic stage. The result suggests conserved evolutionary pathway and similar angiogenic process between vertebrates and cephalopods.

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