CYCLOSTOMI, si-klos'to-me (Gr. around mouthed"), one of the five primary divisions or classes of the craniate Vertebrate, also called Agnatka, Marsipobranckii, etc. Excluding the doubtful extinct ostracoderms, the cyclostomes have an imperfect, embryonic brain case, no lower, jaw, no paired limbs or their supporting girdles, no ribs and no scales. They have a large number of gill pouches, which, have a skeleton not homologous with that of the true fishes. The nostril is single, and the rudiments of the pineal and parietal eyes are remarkably perfect. There is no trace of a swim-bladder. The mouth is armed with a peculiar rasping apparatus. The skeleton is purely cartilaginous and the notochord persists in living forms. There are many remarkable structural special izations in the representatives of the three or four subdivisions, and the peculiarities of the living forms are such as to indicate that they are survivals of a once extensive group of fish like animals; but only a single species of fossil can be referred to here with any certainty. The following may be indicated as orders: Cyclic, with the vertebral column well developed and, like other parts of the internal skeleton, ossified; no external skeleton and no paired limbs; the tail provided with a large diphycercal fin supported by rays; the skull a cartilaginous capsule, with prominent ear sacs, and a large median nasal sac with a circle of cirri about its orifice; this group is represented by a single species (Palcospondylus gunni), from the De vonian rocks of Scotland; Hyperoartia, having the internal skeleton entirely unossified, with a persistent notochord and no vertebra; body eel-like with a caudal fin; the mouth a suctorial disc, with the rasping end of a piston-like tongue appearing at its centre; the nasal sac median and its diverticulum not penetrating the palate; and seven pairs of pouch-like gill slits, whiCh communicate with a common respiratory tube distinct from the oesophagus; represented by the single family Petromyzontida, with the principal genera Petromyzon (lamprey, q.v.),
Lampetra, Mordacia and Geotrsa; and Hyper otretia, like the last in the characters enumer ated except that the nasal diverticulum (ky pophysis) perforates the palate and opens into the mouth and there is no separate internal respiratory tube. There are two families of this group, the Bdellostomidc, with 6 to 14 gill pouches on each side opening separately at the surface; and the Myzinida, with the single genus Myxine (hag fishes, q.v.), in which the six pairs of gill pouches have a single common external opening on each side. To these Pro fessor Cope has added the great extinct group Ostracopkori (q.v.).