In the lowest of the fish-like forms, the lancelet, the eye is simply a minute pigment-spot situated in the anterior wall of the ventricle at the anterior end of the central nervous system. In the hagfishes, which stand next highest in the series, the eye, still incomplete, is very small and hidden by the skin and muscles. This condition is very different from that of the blind fishes of the higher groups, in which the eye is lost through atrophy, because in life in caves or under rocks the function of seeing is no longer necessary. =The Organs of Hearing.=--The ear of the typical fish consists of the labyrinth only, including the vestibule and usually three semicircular canals, these dilating into sacs which contain one or more large, loose bones, the ear-stones or otoliths. In the lampreys there are two semicircular canals, in the hagfish but one. There is no external ear, no tympanum, and no Eustachian tube. The ear-sac on each side is lodged in the skull or at the base of the cranial cavity. It is externally surrounded by bone or cartilage, but sometimes it lies near a fontanelle or opening in the skull above. In some fishes it is brought into very close connection with the anterior end of the air-bladder. The latter organ it is thought may form part of the apparatus for hearing. The arrangement for this purpose is especially elaborate in the carp and the catfish families. In these fishes and their relatives (called _Ostariophysi_) the two vestibules are joined in a median sac (_sinus impar_) in the substance of the basioccipital. This communicates with two cavities in the atlas, which again are supported by two small bones, these resting on a larger one in connection with the front of the air-bladder. The system of bones is analogous to that found in the higher vertebrates, but it connects with the air-bladder, not with an external tympanum. The bones are not homologous with those of the ear of higher animals, being processes of the anterior vertebræ. The tympanic chain of higher vertebrates has been thought homologous with the suspensory of the mandible. [Illustration: FIG. 88.--Brook Lamprey, _Lampetra wilderi_ Jordan and Evermann. (After Gage.) Cayuga Lake.] The otoliths, commonly two in each labyrinth, are usually large, firm, calcareous bodies, with enamelled surface and peculiar grooves and markings. Each species has its own form of otolith, but they vary much in different groups of fishes. [Illustration: FIG. 89.--European Lancelet, _Branchiostoma lanceolatum_ (Pallas). (After Parker and Haswell.)] In the Elasmobranchs (sharks and rays) and in the Dipnoans the ear-sac is enclosed in the cartilaginous substance of the skull. There is a small canal extending to the surface of the skull, ending sometimes in a minute foramen. The otoliths in these fishes are soft and chalk-like.