
790 THE AMERICAN NATURALIST. [Vor. XXXVI. 
A branch of the posterior aorta brings arterial blood to the 
tissues of the branchial heart. 
The aortæ and the larger arteries are peristaltically con- 
tractile and thick walled. Their walls are formed by a thick 
sheet of peculiar muscular tissue, which is enclosed by two 
thin coats of connective tissue. The muscular tissue is formed 
of fusiform cells whose large oval nuclei have a small quantity 
of protoplasm at each end, but the greater portion of each cell 
consists of radiating fibers which interlace with those of neigh- 
boring cells. The muscle resembles connective tissue more 
than muscle, but it is actively contractile, for stimuli can be 
só applied that the peristaltic wave will move in the opposite 
direction to the blood current, and the peristalsis continues long 
after the final stoppage of the hearts. 
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Fic, 2. — Arterioles which show shed effect of the contraction of the scattered intrinsic 
scle fibers of the vessels. 
The muscular coat of the smaller arteries 1s much thinner, 
and the peculiar muscle cells are replaced by simple fibers, 
which are so irregularly distributed upon the smallest arteries 
that their contraction makes the vessels moniliform, as is shown 
in Fig. 2. Arterioles arising as lateral branches of an artery 
commonly have a strong band of muscle fibers at their origin. 
The aortz and their largest branches may have an endo- 
thelium, but the unavoidable contraction of their muscular 
walls throws the inner surface into longitudinal wrinkles, 
between which the silver is deposited in streaks that obscure 
the cell outlines. It is probable that an endothelium is present 
in these vessels ; for, although almost all the evidence is nega- 
tive, a few preparations seem to show the endothelium. The 
^ intermediate arteries are lined | by an endothelium composed of 
