PISCES. | 
Skull of Sea Lamprey ( Petromyzon marinus). After J. Muller. 
@, cartilaginous spinal axis; b, c, rudimentary cartilaginous neural arches ; 
d, d, part of cartilaginous respiratory framework; g, auditory capsule; h, cra- 
nium; #, @’, 7, processes therefrom giving attachment to the muscles of the 
tongue; &, nasal capsule ; /, cartilage covering hinder part of mouth ; n, 0, p,q, 
anterior cartilages entering into composition of mouth ; 7, s, os hyoides. 
nous framework situated on the sides of the 
neck, enclosing the branchial apparatus, and 
allowing respiration to be accomplished by a 
peculiar mechanism which will be described in 
its proper place. This singular structure con- 
sists of seven pairs of cartilaginous arches de- 
rived from a kind of sternum situated in the 
mesial line beneath the throat. These arches, 
which have nothing in common with the bran- 
chial arches of Fishes, mount upwards inter- 
ruptedly towards the spine, giving off anterior 
and posterior processes to form a kind of frame 
to the branchial orifices. Anteriorly this re- 
markable apparatus is attached to the cranium, 
while posteriorly it terminates in a thin cartila- 
ginous capsule, which encloses the heart; it 
will, however, be better to describe it further on. 
In Ammocetes the skeleton is even more im- 
perfect than in the Lamprey, all its parts remain- 
ing permanently in a membranous condition, so 
that they would seem to resemble worms more 
than vertebrated animals, and in fact were ab- 
solutely classed as worms even by the great 
Linneus. 
Skeleton of Branchiostoma.—In this  re- 
markable Fish the entire spine is made up of 
a succession of very delicate membranous rings 
without any apophyses whatever; neither in 
young specimens is the slightest trace of a 
cranial dilatation of the vertebral column appa- 
rent, probably owing to its being at this period 
quite gelatinous in its texture, and conse- 
quently translucent; and even in adults such 
is its softness that it is impossible to distin- 
guish it satisfactorily ; but along the back from 
sixty to seventy vertebre are easily counted, 
the divisions between them being indicated by 
slight bulgings and lines passing obliquely 
from above downwards on the sides of the 
column. In this way a separation between 
the rachidian rings is rather indicated than 
proved to exists for, although there is, so to 
express it, a tendency to divide at the points 
indicated, the division is rather artificial than 
hatural. 
According to Mr. Goodsir, the chorda dor- 
Salis is formed externally of a fibrous sheath, 
967 
and internally of an 
immense number of la- 
mine, each of the size 
and shape of a section 
of the column at the 
place where it is situ- 
ated. When any por- 
tion of the column is 
removed, these plates 
may be pushed out of 
~ their sheath like a pile 
of coins. They have 
no great adhesion to 
one another, are of the 
consistence of parch- 
ment, and appear like 
flattened bladders, as if 
formed of two fibrous 
membranes pressed to- 
gether. 
Two ligaments may 
be detected, one run- 
ning along the upper, the other along the lower 
aspect of the spinal column; and from its 
sides aponeurotic lamine pass off to form 
septa of attachment between the layers of mus- 
cles. Along the mesial plane above the co- 
lumn a similar aponeurosis separates the supe- 
rior lateral muscular masses, and by splitting 
inferiorly, so as to join the sides of the rachi- 
dian chord, forms the canal for the spinal 
medulla. Foramina exist all along this canal 
for the passage of the nerves. A similar apo- 
neurotic septum is situated along the inferior 
part of the column from the anal opening to 
the extremity of the tail. Imbedded in these 
two aponeuroses are cartilaginous rudiments 
representing the superior and inferior vertebral 
spines, but these are of extreme softness and 
delicacy; and the traces which exist of the 
transverse processes and ribs are in the same 
soft condition, so that they are with difficulty 
distinguishable. In Branchiostoma, therefore, 
the locomotive skeleton may be said to consist 
of the vertebral column only, without either 
cranium or appended limbs. 
There is, however, an exceedingly elaborate 
framework of soft cartilaginous arches which 
surrounds the branchial chamber, and forms 
a kind of branchial thorax, the nature of which 
will be examined further on. 
Arthrodial system.—The articulations of the 
bones of Fishes present the same varieties as 
those of other animals; only the arthrodial 
and ginglymoid are more rarely met with 
because their limbs have not to execute such 
complicated movements. 
It is by means of a ginglymus that the 
lower jaw and operculum are attached to the 
pterygo-palatine apparatus, and the latter to the 
cranium. The same articulation occurs be- 
tween the rays of the dorsal and anal fins with 
the interspinous bones, and between the first 
ray of the pectoral fin with the bone analogous 
to the radius. 
There are, moreover, in Fishes two kinds of 
articulations having determinate movements 
which are not met with in the other classes: 
one is formed by two rings joined one to the » 
