Craniate with well-defined heads, thus excluding members of the chordate subphyla Tunicata (tunicates) and Cephalochordata (lancelets), but including Myxini, which have cartilaginous skulls and tooth-like structures composed of keratin. Craniata also includes all lampreys and armoured jawless fishes, armoured fish, sharks, skates, and rays, and teleostomians: spiny sharks, bony fish, lissamphibians, temnospondyls and protoreptiles, sauropsids and mammals. The craniate head consists of a brain, sense organs, including eyes, and a skull. In addition to distinct crania (sing. "cranium"), craniates possess many derived characteristics, which have allowed for more complexity to follow. Molecular-genetic analysis of craniates reveals that, compared to less complex
Craniate animals, they developed duplicate sets of many gene families that are involved in cell signaling, transcription, and morphogenesis (see homeobox). In general, craniates are much more active than tunicates and lancelets and, as a result, have greater metabolic demands, as well as several anatomical adaptations. Aquatic craniates have gill slits, which are connected to muscles and nerves that pump water through the slits, engaging in both feeding and gas exchange (as opposed to lancelets, whose pharyngeal slits are used only for suspension feeding). Muscles line the alimentary canal, moving food through the canal, allowing higher craniates such as mammals to