306 
COMPARATIVE ZOOLOGY. 
Fig. 279. — Diagram of Circulation in 
the higher Vertebrates: 1, heart; 2, 
lungs; 3, head and upper extremities; 
4, spleen; 5, intestine; 6, kidney; 7, 
lower extremities ; 8, liver. (From 
Dalton’s “Physiology.”) 
by a more highly developed 
vertebral column of cartilage 
or bone. The column and 
cranium are never absent in 
the craniota ; other parts may 
be wanting, as the ribs in Frogs, 
limbs in Snakes, etc . 154 The 
limbs are never more than 
four, and are always articu¬ 
lated to the haemal side of the 
body, while the legs of Inver¬ 
tebrates are developed from 
the neural side. The muscles 
moving the limbs are attached 
to the endoskeleton. 
The circulation of the blood 
is complete, the arteries being 
joined to the veins by capil¬ 
laries, so that the blood never 
escapes into the visceral cav¬ 
ity as in the Invertebrates. 
All have a portal vein, carry¬ 
ing blood through the liver; 
all have lacteals and lym¬ 
phatics. The blood is red, 
and contains both kinds of 
corpuscles . 165 The teeth are 
developed from the dermis, 
never from the cuticle, as in 
Mollusks and Articulates; the 
jaws move vertically, and are 
never modified limbs. The 
liver and kidneys are always 
present. The respiratory or¬ 
gans are either gills or lungs, 
