HAG-FISHES. 
1203 
nerves (l), which, after crossing, emerge from the vent- 
ral side of the midbrain and afterwards turn first out- 
wards and then forwards, penetrate the hind extremity 
of either eye, and their fibres may be traced some way 
in within its central parts. The eyes, surrounded by 
a capsule of connective tissue, do not show the regular 
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 present the nor- 
mal arrangement, it fills nearly the whole orbit, its 
stratification is very irregular and difficult of elucida- 
tion, instead of rods and cones there is only an epi- 
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 anterior end of the notochord at the 
bottom of the cranium (fig. 355, c ). In the annular 
tube may be distinguished an inner division (tig. 360, a), 
furnished with a long nerve-ending, a macula acustica 
(e), and an outer division (5), furnished at each end 
with an ampullaceous dilatation (c, d) and a nerve- 
ending, thus with two cristas acustica? (/, g). This 
outer division may reasonably be interpreted as repre- 
senting two semicircular canals, each with an ampulla. 
The inner division gives out a ceecally terminating en- 
dolymphatic duct ill). 
It is else not easy to institute any close comparison 
between the so little differentiated parts of the auditory 
organ in Myxine and those of the corresponding organ 
in other fishes and the higher vertebrates. Unfortuna- 
tely this applies also to the same organ in Petromyzon , 
where it has many peculiarities and forms no true link 
c b 
e' d 
Fig. 361. Auditory organ (right) of a Lampern ( Petromyzon fluviatilis ), X 18. 
A, seen from below; B, seen from above and without: a, a 1 , vestibule; 6, anterior semicircular canal; c, c\ anterior ampulla; d, posterior 
semicircular canal; e, e 1 , posterior ampulla; /, saccate appendage; f 1 , its nerve-ending; fa, commissure; g , ganglion of the acoustic nerve; h , acoustic 
nerve; i, cerebral part from which the acoustic nerve originates; k, anterior branch of the acoustic nerve; l , posterior branch of the acoustic nerve. 
Myxine is not only undeveloped, but 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 
of appreciating with these rudimentary eyes certain 
intense lights. 
The auditory organ of Myxine (fig. 360) is struc- 
turally the simplest known within the vertebrate king- 
dom, if we except Branchiostoma , 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 Petromyzon (fig. 361) may be distinguished a large 
common chamber (the vestibule), a saccate appendage, 
and two semicircular canals each with an ampulla 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, C , B) that the membranous auditory or- 
gan is developed to that fundamental type 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- 
