Parasitic flatworms (schistosomes) live in the human bloodstream and are covered by a skin-like structure called the tegument, which is constantly replenished by newly produced stem cells (see inset) and so protects the worms from being detected by the host immune system. The tegument (blue) is made of cells that have fused together to form a continuous structure and express a protein called TSP-2 (purple). The cells in the tegument derive from stem cells (orange). These stem cells divide to produce intermediate cells (yellow), which mature to produce the cells (blue) that fuse to form the tegument. These tegument-fated intermediate cells express ‘zinc-finger’ proteins (zfp) and are necessary to build the skin-like structure. Creating drugs that could block these genes may present a new opportunity to treat schistosomiasis.