1883. ] Especially those of the Limbs. 507 
We get but little information on this point from the embryo. 
It will therefore be necessary to examine the vascular system in 
those animals along which we suppose the line of descent to have 
passed, getting thereby a fuller history of the successive stages 
than we can do in the condensed abridgment shown us in embry- 
onic life. 
Not until we reach worms do we find the commencement of a 
true d/ood-vascular system. It is true that even Amoebze have a 
trace of vessels formed by the coalescence of vacuoles arising at 
indeterminate points in the protoplasm, but these are not perma- 
nent. In Vermes we have, formed within the layers of the meso- 
blast, permanent vessels having walls and independent of the 
body cavity. | 
In the simplest form we have three longitudinal trunks, two 
lateral, the third medio-dorsal. Simply connected behind, toward 
the cephalic end they are somewhat coiled around the ganglionic 
center which represents the primitive cerebrum, distantly remind- 
ing us of the branchial arches of vertebrates. In some genera 
transverse vessels connect these. This is more distinctly shown 
in the ringed worms where a distinct transverse trunk exists for 
each segment of the body. Some portion of these vessels may 
become pulsatile, sometimes a transverse vessel, sometimes a dor- 
sal one. This is the primitive heart, originally a portion of the 
tubular system which takes on contraction. (Plate IX. figs. a, 4, £.) 
Doubtless trunks were originally developed for the reason that 
fluids will move more rapidly and effectively through a large 
Straight vessel of uniform size than through a capillary network. 
As these worms move their bodies longitudinally, mechanical 
force acting on the blood would tend to enlarge the capillaries in 
that direction, 
Special nervous system and muscles. Hence each has a 
semi-independent center of nutritive activity and a lateral trunk 
F ready and effective supply. Vertebrate animals, including 
man, share with these lowly organisms this peculiar ramification. 
aorta gives off the intercostal arteries which pass around the 
3y and connect with other longitudinal vessels (internal mam- 
