24 



epithelium *. The whole resembles a fine arachnoid membrane (a portion of this peri- 

 cardium is shown at b, fig. 2, PL I.). 



The heart is fusiform, widest at its hinder third, gradually narrowing, transversely, to 

 its fore end, which is 2| inches from the fore part of the cephaletron, more rapidly con- 

 tracting to its hind end, which is 1^ inch from the joint of the tail-spine. In the vertical 

 diameter (PL II a. fig. 1, r, r) the hinder contraction is more gradual. 



The heart-wall consists of an outer, thin, smooth, compact coat, and a waU 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 I. 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 sm-face, 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 II A. fig. 1, r, r) is about a line in thickness, but thins off rapidly at the two ends of 

 the heart. Each ' ostium ' (ib. o, o) 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 I. fig. 2, h) runs to the ' ocelli ' {a 1), is there connected 

 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 II a. fig. 1, s) : it gives off, in this course, a branch 

 Avhich ramifies upon the gizzard, a second to the intestine and liver, the main trimk 

 being continued to the nervous annu.lar centre (ib. /3), where it expands, and combines 

 with its fellow of the opposite side to form a sheath for that centre analogous to a ' dura 

 ]nater.' This rather loose sheath is continued along the ganglionic 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. 



Pine 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. /3) ; 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 timics of the nervous matter of the neural ringt. The same condition was pre- 

 sented by the seeming abdominal arterial trunk continued backward from the neur- 

 arterial circle (PL V. a, B, to or. of nerve n 19). On slitting open the coats of the blood- 



* Owen, ' Anat. of Vertebrates,' vol. iii. p. 499. 



t Preparation, No. 1303 c, Physiol. Series, Mus. Coll. of Surgeons. 



