504 STRUCTURE OF VERTEBRATA. 
It is probably never quite closed, but may communicate with the 
exterior by abdominal pores (or through nephrostomes) opening into 
the renal system. Both occur together in some Elasmobranchs, but 
they are usually mutually exclusive. In the higher Teleostei, in some 
Saurians, and in Mammals, there are neither abdominal pores nor 
nephrostomes, but only openings (stomata) into the lymphatic 
system. 
Vascular system.--From Cyclostomata onwards the 
blood fluid contains red cor- 
puscles, z¢. cells coloured 
with heemoglobin—a pigment 
which readily forms a loose 
union with oxygen, and bears 
it from the exterior (through 
gills or lungs) to the tissues. 
These pigmented cells are 
usually oval and nucleated. 
In all Mammals except 
Camelidz they are circular. 
Moreover, the full-grown red 
corpuscles of Mammals have 
no visible nuclei. The blood 
fluid also contains uncoloured 
Fic. 268.—Blood corpuscles. 
x. Amphibian, seen on the flat, oval, 
bi-convex disc (nucleated); 2, am- 
phibian, in profile; 3, mammalian 
(non-nucleated), circular, bi-concave 
disc; 4, mammalian, in profile; 5, 
camel’s (non- nucleated), oval; 6, 
mud-fish (Lepidosiren) in_ section, 
like Amphibian ; 7, Lepidosiren, seen 
on the flat ; 8, an amoeboid leucocyte 
with lobed nucleus and large gran- 
ules; 9, a leucocyte with non-lobed 
nucleus and minute granules; 10, a 
leucocyte dividing into two; 11, a 
flat amoeboid corpuscle or blood 
platelet 
inequality of pressure which makes the blood flow. 
nucleated amceboid cells, the 
white corpuscles or leuco- 
cytes, of much physiological 
importance. Some of them, 
specialised as phagocytes, 
form “a body-guard,”. at- 
tacking and destroying micro- 
organisms within the body. 
The heart receives blood 
from veins, and drives it forth 
through arteries. Its contrac- 
tions in great part cause the 
It lies 
in a special part of the body cavity known as the peri- 
cardium, and develops from a single (sub-pharyngeal) 
vessel in Cyclostomata, Fishes, and Amphibians, from a 
pair in Reptiles, Birds, and Mammals. 
The receiving region of the heart is formed by an auricle 
or by two auricles; thence the blood passes into the 
