Body is cylindrical with a laterally compressed tail. The head is long. The body is covered with thick skin and secretes mucus, the skin has many and dense pigmented cells. No scales on the body surface. Mouth large; teeth very sharp and uniquely arranged; no tongue. Posterior nostrils round, tubular or short tubular. Gill slits foraminal; fourth pair of gill arches thickened and specialized, connected to pharyngeal cephalic plates located above and below the esophagus, forming a bony structure that aids pharyngeal feeding. Pectoral and ventral fins absent; dorsal fin base begins in front of gill pores; dorsal and anal fins connected to caudal fin. Body color and spots vary greatly, including monochromatic, fine dots, dots, irregular patterns and stripes, which is an important basis for the classification of fishes in this family.