1180 
SCANDINAVIAN FISHES. 
at 
Fig. 350. Section of the forepart of a Lampern ( Petroinyzon ); about 
twice the nat. size; partly after Pakkek, partly after Vogt and Yung. 
abr , branchial artery; ally , anterior hyoid cartilage; aln, anterior 
linguo dental plate; alt , anterior lateral dental plate; anti, annular car- 
tilage of the mouth (severed in two places); at, anterior dental plate; 
aurc, auricle of the heart; c, ventricle; Cer I + II, prosencephalon 
and tlialamencephalon; Cer III , mesencephalon; Cer IV + V, cerebellum 
and medulla oblongata; cli, chorda dorsalis; coel, abdominal cavity 
(coelom); ctr, cornua trabecularum; clc, ductus Cuvieri; ebr, outer gill- 
openings; hep, liver; Hy, nostril; ibr I — VII, first-seventh inner gill- 
openings; med, median upper cartilage of the mouth; Ml, myelon; 
mlt, left mediolateral dental plate; myc, myocomma; mym, myomere; 
n, olfactory capsule (the severed cartilaginous wall); oes, oesophagus; 
ovar, anterior extremity of ovary; pliy, posterior hyoid cartilage; pt, 
posterior dental plate of the mouth ; t, tegmen cranii ; v, anterior 
part of intestine; vel, velum. 
paired eyes is connected, and its manner of origin is 
the same, the difference is strictly but local. In its 
highest development too, it becomes an eye with optic 
nerve, an eye in the middle of the skull, asymmetrical, 
for its nerve root would appear to originate from the 
left optic thalamus, though before this becomes a true 
optic thalamus, and while it still lies, as in the Lampreys, 
in the roof of the brain and bears the name of ganglion 
habenulce (intermedium) sinistrum. This pineal eye, 
whose functional time proper fell under the period known' 
by geologists as the Mesozoic, bore within itself, how- 
ever, the seeds of its destruction in the vertebrates, for 
it was constructed on the ocular type of the invertebrates: 
— its lens was an ependymal instead of an epithelial 
(epiblastic) growth, and its retinal cells had the base 
directed peripherally instead of centripetally into the 
eyeball. And after the advent of the Tertiary period this 
Polyphemous type disappeared from among the verteb- 
rates. In many, however, as in the Lampreys, it endures 
in a more or less vestigial condition. To all appearances 
the pineal eye was more ancient than the paired eyes, 
perhaps originally the only true eye in the worm-like 
ancestors of the vertebrates, hardly elevated as yet above 
the level of the invertebrates; and all the sensations des- 
tined for our consciousness still follow in the optic thalami 
(if we may believe Luys, fig. 348) a path laid down 
through the realm where the pineal eye once held sway. 
The mental faculties of the higher vertebrates 
had a long history before they became what they 
are. The pineal eye and the sensory organs of the 
primitive gill-clefts are examples of structures at dif- 
ferent grades of abortion. The first has almost ceased 
to functionate at all, and terminates its existence as 
a functional organ in the class of reptiles; the latter 
do not extend so high in the animal series, and they 
nowhere retain their original, preponderating signifi- 
cance; but they have a descendant, the system of the 
