This slide shows a stained cross section through the upper pharyngeal region of the lancelet Amphioxus).  Observe the dorsal fin supported by a fin ray.  Note the conspicuous myomeres (muscle blocks) that are separated by myosepta.  Above the prominent notochord is the hollow dorsal  nerve cord.  The two-layered skin of the lancelet is composed of an outer epidermis of simple columnar epithelium and an inner dermis of gelatinous connective tissue.