The development of the auditory organs in the Crustacea has not been investigated.
The Vertebrata. The Cephalochorda are without organs of hearing, and the auditory organ of the Urochorda is constructed on a special type of its own. The primitive auditory organs of the true Vertebrata have the same fundamental characters as those of the majority of aquatic invertebrate forms. They consist of a vesicle, formed by the invagination of a patch of epiblast, and usually shut off from the exterior, but occasionally (Elasmobranchii) remaining open. The walls of this vesicle are always much complicated and otoliths of various forms are present in its cavity. To this vesicle accessory structures, derived from the walls of the hyomandibular cleft, are added in the majority of terrestrial Vertebrata.
The development of the true auditory vesicle will be considered separately from that of the accessory structures derived from the hyomandibular cleft.
Fig. 299. Section through the head of an Elasmobranch embryo, at the level of the auditory involution.
aup. auditory pit; aun. ganglion of auditory nerve; iv.v. roof of fourth ventricle; a.c.v. anterior cardinal vein; aa. aorta; I.aa. aortic trunk of mandibular arch; pp. head cavity of mandibular arch; Ivc. alimentary pouch which will form the first visceral cleft; Th. rudiment of thyroid body.
In all Vertebrata the development of the auditory vesicle commences with the formation of a thickened patch of epiblast, at the side of the hind-brain, on the level of the second visceral cleft. This patch soon becomes invaginated in the form of a pit ([fig. 299], aup), to the inner side of which the ganglion of the auditory nerve (aun), which as shewn in a previous chapter is primitively a branch of the seventh nerve, closely applies itself.
In those Vertebrata (viz. Teleostei, Lepidosteus and Amphibia) in which the epiblast is early divided into a nervous and epidermic stratum, the auditory pit arises as an invagination of the nervous stratum only, and the mouth of the auditory pit is always closed ([fig. 300]) by the epidermic stratum of the skin. Since the opening of the pit is retained through life in Elasmobranchii the closed form of pit in the above forms is clearly secondary.
In Teleostei the auditory pit arises as a solid invagination of the epiblast.
The mouth of the auditory vesicle gradually narrows, and in most forms soon becomes closed, though in Elasmobranchii it remains permanently open. In any case the vesicle is gradually removed from the surface, remaining connected with it by an elongated duct, either opening on the dorsal aspect of the head (Elasmobranchii), or ending blindly close beneath the skin.