VERTEBRATA. 347 



the anterior or inner angle of the eye. The auditory organ commences 

 as an epiblastic involution. It becomes saccular, but the pedicle of invagi- 

 nation persists as the aquaeductus vestibuli, or saccus endolymphaticus, 

 which in some instances, Lacertilia especially, becomes much enlarged, 

 and in many Elasmobranchii remains open externally. The ear-sac 

 differentiates into two connected vesicles, the utricle and saccule. Three 

 semicircular canals, anterior vertical, posterior vertical, and external 

 horizontal, originate from the utricle ; a lagena, or cochlea, from the 

 saccule. The whole constitutes the membranous labyrinth. The auditory 

 nerve is distributed to certain spots, the maculae of the utricle, saccule, 

 and cochlea, the cristae acusticae of the semicircular canals, the epithelium 

 of which consists of sense-cells and supporting cells. The whole apparatus 

 is filled with a liquid, or endolymph. Calcareous otoliths are found in the 

 sensory region of the maculae, except in the cochlea of Mammalia. They 

 vary in size and are borne by the sense hairs of the epithelium, and when 

 small are united by a slimy substance. The membranous labyrinth is 

 inclosed in the auditory capsule, but is separated from it by a space filled 

 with perilymph. In Pisces this space opens into the arachnoid space of 

 the brain by passages round the auditory nerve and the aquaeductus 

 vestibuli, but by a special duct the aquaeductus cochleae (ductus peri- 

 lymphaticus) in Amphibia and higher Vertebrata. Two membranous spots, 

 the fenestra ovalis and fenestra rotunda, occur on the outer wall of the 

 auditory capsule, where it bounds the perilymphatic space. They are 

 found in most Amphibia and all higher Vertebrata. To the first is attached 

 either a single auditory ossicle, or a chain of ossicles, which connect it 

 to the tympanic membrane (infra). The single ossicle is the stapes 

 ( = hyomandibular of Pisces), found in Amphibia and Sauropsida, where it is 

 often broken up into a series of segments (supra, p. 339), and is generally 

 known as columella auris. The chain of ossicles is found in Mammalia, 

 and comprises a stapes, articulated to an incus ( = quadrate of Sauropsida, 

 &c.), and that in turn to a malleus (= articular part of Meckel's arch). 

 These bones are contained in Anuran Amphibia and higher Vertebrata 

 in a tympanic cavity, produced by an outgrowth from the pharynx in 

 the position of the first visceral cleft or spiracle, which atrophies almost 

 completely. The part opening into the pharynx is the Eustachian tube. 

 The outer wall of the outgrowth, together with mesoblast and epiblast, 

 forms the tympanic membrane to which one end of the columella auris, 

 or in Mammalia the malleus, is attached. An external meatus is formed 

 in higher Vertebrata by the growth of the parts around the tympanic 

 membrane, but a concha of the ear is well developed only in Mammalia. 



The alimentary tract consists of a stomodaeum, mesenteron (= archen- 

 teron), and a proctodaeum, but in some Amphibia (Frog, Newt) the 

 blastopore persists as the anus. The stomodaeum forms the major part of 



