OF THE AMERICAN KING-CRAB. 481 
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 
epithelium *. The whole resembles a fine arachnoid membrane (a portion of this peri- 
cardium is shown at 5, fig. 2, Pl. XXXVI.). 
The heart is fusiform, widest at its hinder third, gradually narrowing, transversely, to 
its fore end, which is 23 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. XXXVII. fig. 1, 7, 7) the hinder contraction is more gradual. 
The heart-wall consists of an outer, thin, smooth, compact coat, and a wall of striate 
muscular fibres consisting of a thin outer longitudinal layer and a thicker transverse or 
circular series. The wider part of the heart shows traces of an epithelial lining, due to 
the action initiated or invited by a surface in contact with the formifying material in 
solution. The arachnoid coat of the pericardial sinus is reflected over the outermost 
proper tunic of the heart, and is continued into the venous ostia (Pl. XX XVI. fig. 2, c”, c), 
where it gains thickness. 
These ostia are sixteen in number, arranged in seven pairs at the sides, but towards 
the dorsal surface, of the heart, with a terminal eighth pair. The hinder “ostia” are 
rather nearer together than the others. The muscular tunic in the intervals of the ostia 
(Pl. XXXVII. fig. 1, 7, 7) is about a line in thickness, but thins off rapidly at the two 
ends of the heart. Each * ostium’ (ib. o, 0) is provided with a pair of narrow semilunar 
valves, placed with the intermediate slit almost transversely to the axis of the cardiac tube. 
The foremost artery (Pl. XX XVI. fig. 2, h) runs to the * ocelli’ (a 1), is there con- 
nected with, or seems to enclose, the nerve; it then bends down, following the curve of 
the carapace to the angle formed by the upper with the flat under surface of the digging- 
shield, near which angle the artery is reflected backward and cannot be further traced as 
a distinct tube. On each side the origin of the * ocellar’ artery arises one of double the 
size (ib. e, e), which, diverging from its fellow, curves outward and downward over the 
fore part of the intestinal canal (Pl. XXXVII. fig. 1, s): it gives off, in this course, a 
branch which ramifies upon the gizzard, a second to the intestine and liver, the main trunk 
being continued to the nervous annular centre (ib. 8), where it expands, and combines 
with its fellow of the opposite side to form a sheath for that centre analogous to a ‘ dura 
mater.” This rather loose sheath is continued along the ganglionie ventral chord, and 
is prolonged, like a loose neurilemma, upon the nerves sent off therefrom, as it is upon 
those in connexion with the annular centre. 
Fine size-injection being thrown into the ‘heart’ from behind forward, appeared to 
give a rich display of arterial ramifications. But dissection showed that the contents of 
the seeming arteries ceased to be the simple injected matter where the * gastric arches ' 
(ib. s) reached the nervous ring (ib. 8); for here the coats of the artery become thinned, 
the injection lining them as a thin flake of colouring-matter, and, at the same time. 
covering a thinner membrane which formed the * neurilemma,' or chief layer of the deli- 
cate tunics of the nervous matter of the neuralringt. The same condition was presented 
* Owen, * Anat. of Vertebrates,’ vol. iii. p. 499. 
+ Preparation, No. 1303 c, Physiol. Series, Mus. Coll. of Surgeons. 
VOL. XXVIII. 3 T 
