296 COMPARATIVE ZOOLOGY. 
Fig, 271.—Diagram of Circulation in 
the higher Vertebrates: 1, heart; 2, 
lungs; 8, 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; 
other parts may be wanting, 
as the ribs in Frogs, limbs in 
Snakes, etc. The limbs are 
never more than four, and 
are always articulated to the 
heemal side of the body, while 
the legs of Invertebrates 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 cay- 
ity as in the Invertebrates. 
All have a portal vein, car- 
rying blood through the liv- 
er; all have lacteals and lym- 
phatics. The blood is red, 
and contains both kinds of 
corpuscles." 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, 
