The Collaborative International Dictionary of English v.0.48:

Cephalosome \Ceph"a*lo*some\ (s[e^]f"[.a]*l[-o]*s[=o]m, n.
   [Cephalo- + -some body.] (Zool.)
   The anterior region or head of insects and other arthropods.
   --Packard.
   [1913 Webster]