In Limulus the walls of this vasiform heart exhibit muscular and valvular structures,
for the same purpose or office as those of the vertebrate ‘ heart.’

In a specimen dissected, with a carapace, or upper crust of the two chief parts of the
body, 9 inches in length, the heart was 4 inches 8 lines in length (PI. I. fig. 2, a, a). It
was included in a delicate membranous sac analogous to a pericardium, but forming,
in fact, the wall of a venous sinus. This wall consists of two layers. One may be
properly termed a ‘tunic:’ it includes extremely delicate fibres, chiefly transverse, but
reticularly interwoven in a fine cellular bed, the inner surface of which has been the seat
of formifaction, or vital crystallization, of particles from the contained fluid, forming an

+ a, fig. 109, « Lectures on Invertebrata,’ 8yo, 1843.
24


epithelium *. The whole resembles a fine arachnoid membrane (a portion of this peri-
cardium is shown at 8, fig. 2, Pl. I.).
The heart is fusiform, widest at its hinder third, gradually narrowing, transversely, to
its fore end, which is 24 inches from the fore part of the cephaletron, more rapidly con-
tracting to its hind end, which is 13 inch from the joint of the tail-spine. In the vertical
diameter (Pl. IT a. fig. 1, 7, 7) the hinder contraction is more gradual.
