ANIMAL — CIRCULATORY SYSTEM. 
189 
which is considered to be a secretion of the epithelinin of the intes- 
tinal tract. The intestines are always considerably longer than the 
body cavity containing them, and are therefore necessarily thrown 
into a number of coils or convolutions, chietly amongst the lobules ot 
the liver, the absorjitive surface being often greatly increased by a 
longitudinal infolding of its surface, called the typhlosole. 
The Circulatory system occupies the closed space or cavity 
between the alimentary canal 
and the external integument, 
and is formed by definite vessels 
in conjunction with a complex 
Fig. 296. — A fragment of subfilamentar lacunar 
or primitive mesohlastic tissue in Anodotita^ 
showing the trabeculcc and their nuclei (after 
Holman Peck), cor. amceboid blood corpuscle, 
ep. epithelium, nc. nucleus surrounded bygranular 
protoplasm, tr. trabecula showing peripheral e.\- 
pansion. 
system of irregular spaces or 
lacuna, which permeate among.st 
the various organs and within 
the interspaces excavated among 
the viscera, the muscular and 
connective tissue. 
A dorsally placed heart or 
central motor of circulation is always ju esent enclosed within a special 
chamber or pericardium, and is always arterial or systemic, the 
auricles receiving oxygenated 
blood from the respiratory 
organs and propelling it by 
means of a muscular ventricle 
through the body system. 
The blood or Inemolymph, 
which forms a large proportion 
of the total weight of the body, 
is usually a colourless or slightly 
opalescent albuminous fluid, 
containing numerous nucleated amoeboid corpuscles, which are shed 
from the walls of the coelomic space. 
By far the gTeater number of mollusca in general respire by means 
of branchial or gills, and a few like Planorhis combine branchial and 
pulmonary respiration, but species with solely aerial respiration are 
confined to the Gastropoda. Respiration, however, is not confined to 
special organs, but is participated in by the general integument and 
more especially by the surface of the mantle, which is of a very 
vascular character. 
Fig. 297. — Ama;boid blood corpuscles of 
Helix poinatia^ highly magnified (after Vogt 
and Yung). 
