It means he doesn't have a skull, he doesn't have a brain coated with a box of bones. It is the name of marine animals also called leptocardia, cephalodords or amphioxes. They live on sandy bottoms, are elongated and the head is long and rope-shaped.
acranium-refers to the expression : small cephalogards, elongated, transparent, compressed, and sharpened at its two ends, with a long dorsal fin-