596 COMPARATIVE ANATOMY 



quite unable to maintain its balance in swimming. Plate's suggestion 

 that the ciliated infundibular organ of amphioxus is a static organ has little 

 in its favor. 



All vertebrates have a static organ, which is a novelty in this group 

 and not a structure inherited from invertebrate forbears. In fishes and 

 aquatic urodeles, the membranous labyrinth of the ear appears to be 

 exclusively a static organ. But in land forms, beginning with the 

 amphibia, the ear has the double function of equilibration and hearing. 



Of vertebrates, the cyclostome myxine has the simplest static organ, 

 which in shape is not unlike an inflated inner tube of an automobile tire 

 with some inequalities of expansion. From its nerve supply, it is thought 

 to correspond to the utriculus and the two vertical semicircular canals 

 of the higher vertebrates. Homology with two semicircular canals of 

 vertebrates rests on the presence of two ampullae, each of which contains 

 an elongated cluster of hair cells, the crista, innervated by branches of the 

 auditory nerve. Each of the semicircular canals of higher vertebrates 

 has, however, only a single crista and ampulla, with three sensory maculae 

 representing the single macula of myxine. A macula is a cluster of sensory 

 cells with short hairs located either in the utriculus or sacculus. A rudi- 

 mentary endolymphatic duct which extends dorsally towards the skin is 

 present. 



The statocyst of petromyzon is slightly more complex than that of 

 myxine. A ventral sacculus is partly separated from the utriculus by a 

 circular constriction. There are three maculae instead of one. The 

 statolith is represented by a mass of calcareous particles encased in a 

 mucous matrix, and lying in contact with the hairs of the sensory cells of 

 the maculae. None of the cyclostomes has a horizontal semicircular canal. 



In elasmobranchs, the cavity of the statocyst retains its primary 

 connexion with the outside, through the persistent invagination canal, 

 which is sometimes erroneously homologized with the endolymphatic 

 duct of higher vertebrates. The statocyst is filled with sea water, instead 

 of endolymph secreted by its own cells; and, in some species of sharks and 

 rays, grains of sand replace calcareous statoliths. All three semicircular 

 canals are present, as in higher vertebrates. The sacculus becomes 

 further separated from the utriculus, and connexion between the two is 

 effected by a canalis reuniens. The lagena is formed as an outgrowth of 

 the sacculus, but it has not been demonstrated that it has an auditory 

 function. The entire membranous labyrinth is enclosed in a cartilaginous 

 capsule, which fuses with the cranium. 



In teleosts, the invagination canal degenerates, and is replaced by an 

 endolymphatic duct, which grows out from the sacculus. The cavity of 

 the bony auditory capsule opens into that of the cranium, and the peri- 

 lymph surrounding the statocyst is identical with the cerebrospinal fluid. 



