IIA(J-KlSincs. 



1 2U3 



nerves (/), wliii-li, after cros.siii;;-, emerge tVoiii tlie vent- 

 ral sifle of tlie inidhi-ain and afterwards turn first out- 

 wards and then forwards, penetrate the hind extremity 

 of eithei- e}'e, and their fibres may be traeed some way 

 in witliin its central parts. The eyes, surrounded by 

 a eajjsule of eonneetive tissue, do not show the regidar 

 structure common in the vertebrates; cornea, iris, and 

 lens are entirely wanting, and of vitreous body there is 

 only a rudiment; the retina does not ))resent tlie nor- 

 mal arrangement, it fills nearly tlic whole orbit, its 

 stratitication is very irregular and ditticult of elucida- 

 tion, instead of rods and cones there is only an cjii- 

 thelial layer of obscure nature. By means of the mo- 

 dern colouring methods the presence of numerous cells 

 of neuroglia-like appearance may be demonstrated, but 

 also of some other cells that may possibly be of a true 

 nervous description. Hence it appears that the eye of 



each side of the ant('ri(jr end of the notocliord at the 

 bottom of tlie cranium (Hg. ;-i.').'), c). In tlie annular 

 tube may be distinguished an inner division (fig. 3(J0, a), 

 furnished with a hjng nerve-ending, a macula acustica 

 (c), and an outer division (h), furnished at eacii end 

 witli an aniiiuilace<jus dilatation (c, d) and a nerve- 

 ending, tiiiis witli two eristic acustica^ (/, </). This 

 outer division may reasonably be interpreted as repre- 

 senting two semicircular canals, each with an ampulla. 

 Tlie inner di\isi(iii gives out a ca'cally terminating en- 

 cloh^m|ihatic duct (li). 



It is else not easy to institute any close compa.ris(jn 

 between the so little differentiated parts of the auditory 

 organ in Myxine and those of the coi-responding organ 

 in other fishes and the higher vertebrates. Unfortuna- 

 tely this applies also to the same organ in Petrnniijzon, 

 where it has many peculiarities and forms no true link 

 c b 



d-^ 



i- h 



A i 



Fig. S6l. Auditory organ (right) of a Lampeni {Petromi/zon fiuoiatilis), X 18. 

 A, seen from below; B, seen from above and without: a, a', vestibule; J, anterior semicircular canal; f, f', anterior ampulla; d, posterior 

 semicircular canal; e, e', posterior ampulla; /, saccate appendage; /', its nerve-ending; fc, commissure; g, ganglion of the acoustic nerve; /i, acoustic 

 nerve; ;, cerebral part from which the acoustic nerve originates; k, anterior branch of the acoustic nerve; /, posterior branch of the acoustic nerve. 



Mijjittc is not only undeveloped, liiit has probably also 

 undergone retrogressive development. That no true sight 

 exists, is evident from the structure of the eye and its 

 concealment below the surface of the body. Yet phy- 

 siological experiments might well be made to investigate 

 whether, in spite of this, the fish may not be capable 

 uf appreciating with these rudimentary eyes certain 

 intense lights. 



The auditnnj offjun of Mijxine (fig. 860) is struc- 

 turally the simplest known within the vertebrate king- 

 <lom, if we except Branchtostoma, in which no trace of 

 such an organ has been discovered. It consists of a 

 paired, annular, membranous organ, which lies enclosed 

 in the above-described cartilaginous capsule situated on 



between that of Myxine and of the other vertebrates. 

 In Petromijzon (tig. 361) may be distinguished a large 

 common chamber (the vestibule), a saccate appendage, 

 and two seinicircidar canals each with an amjiulla and 

 appertaining crista acustica. In the vestibule is a large 

 nerve-ending (macula acustica) perhaps composed of se- 

 veral, and a smaller one appears in the saccate appendage. 

 It is first in the osseous and cartilaginous fishes 

 (fig. 362, A, B, (', I)) that the membranous auditory or- 

 gan is developed to that fundamcnital ty])e which it 

 afterwards retains throughout its evolution within the 

 vertebrate kingdom, even in the human race. It parts 

 into two divisions, a superior and an inferior. The 

 superior consists of a fairly wide, horizontally and sagit- 



