THE FISHES 157 
sand in warm seas, the nine species known being found in 
as many different regions. A lancelet may be regarded as 
a vertebrate reduced to its lowest terms. Instead of a 
jointed back-bone, it has a cartilaginous notochord, running 
from the head to the tail. A nervous cord lies above it, 
enclosed in amembranous sheath. No skull is present, and 
the nerve-cord does not swell into a brain. There are no 
eyes and no scales. The mouth is a vertical slit, without 
jaws. There is no trace of the shoulder-girdle (shoulder- 
blade and collar-bone) or pelvis (hip-bone) from which 
aI. ts 
Aes 
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ae — : i 
Fie. 98.—The California lancelet (Branchiostoma californiense). Twice the natural 
size. g, gills; 2, liver; m, mouth; nm, nerve-cord ; nc, notochord. 
spring the paired fins, which, in true fishes, correspond to 
armsandlegs. The circulatory system is fish-like, but there 
is no heart, the blood being driven about by the contraction 
of the walls of the vessels. Along the edge of the back and 
tail is a rudimentary fin, made of fin-rays connected by mem- 
brane. In the character and arrangement of its organs the 
lancelet is certainly like a fish, but in degree of develop- 
ment it differs more from the lowest fish than the fish does 
from a mammal. 
150. Lampreys (or Cyclostomes).—The class of lampreys 
stands next in development (Fig. 99). The notochord gives 
way anteriorly to a cartilaginous skull, in which is con- 
tained the brain, of the ordinary fish type. There are eyes, 
and the heart is developed, and consists of an auricle and 
a ventricle. As distinguished from the true fish, the lam- 
preys show no trace whatever of limbs or of the bones 
which would support them. The lower jaw is wholly want- 
ing, the mouth being a roundish sucking disk. The fins 
