THE SENSE ORGANS 417 



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 

 rudimentary 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 

 statohth 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 statoHths. 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 cerebro-spinal fluid. 

 The cavity of the otic capsule in amphibians becomes closed and inde- 

 pendent of that of the cranium. Near the lagena, another outgrowth of 

 the sacculus, the basilar papilla, is formed. From this and the lagena, 

 arises the cochlea of the higher vertebrates. To its static function, the 

 ear of the amphibians now adds an auditory one. (Fig. 367) 



The loss of gills in land amphibians is associated with important changes 

 in the visceral arches. The hyomandibular cartilage ceases to be a sus- 

 pensory apparatus of the jaw, and slips into the spiracular passage as the 

 stapes or columella. Amphibians are, therefore, the first animals to add, 

 to the inner membranous ear, structures corresponding to the middle ear 

 of mammals. (Fig. 367) 



