242 
Argas persicm 
The Heart. 
Plates XIV, XV and XVI, ht, Plate XVII, fig. 8. 
The heart is a single-chambered vessel which is situated in the 
upper portion of the body-cavity immediately beneath the dorsal integu¬ 
ment, a shoi’t distance posterior to the level of the spiracles. As seen 
from above, the organ appears as a small transparent flattened sac, 
resting on the dorsal surface of the stomach, and which in the living 
animal exhibits a rapid and regular pulsation. Its contour is more or less 
triangular, the angles being rounded off; and from the anterior angle— 
the apex of the triangle—arises a single large vessel, the cephalic 
aorta (ao.), which runs forwards on the surface of the stomach and dis¬ 
appears from view at the point where the latter organ divides to form 
the two antero-lateral lobes. The caudal artery which Nordenskibld^ 
identified in /. ricinus is not developed in A. p>e7'sicus. 
On account of the exceedingly delicate texture of its walls, and the 
fact that the organ is attached to the surrounding parts by strands of 
muscular and connective tissue, it is a matter of the greatest difficulty 
to remove the heart in its entirety; it is, therefore, most convenient to 
study its structure from serial sections of the entire animal. The walls 
of the heart (see PL XVII, fig. 8) are composed of a thin layer of homo¬ 
geneous protoplasm, covered on both inner and outer surfaces by a 
delicate limiting membrane, within which the muscle fibres {m.ht'.) and 
numerous flattened nuclei are distributed. No cellular divisions can be 
recognised. In the ventral wall, .situated a little towards the anterior 
end of the organ, two slit-like ostia {os. lit.) appear, one on either side of 
the median line; each ostium is guarded by a valvular flap (v. os. lit.) 
which is developed from the posterior margin of the opening and which 
protrudes into the lumen of the heart. The muscle fibres in the heart 
wall are transversely striated, and are, for the most part, disposed in a 
radial manner, extending from the central portion of both upper and 
lower walls towards the periphery of the organ. Other muscle fibres 
surround the margins of tlie ostia and extend to the valvular flaps which 
guard their orifices. In addition to the intrinsic muscles of the heart, 
other muscle strands {m. lit.) extend from the surface of the heart to the 
surrounding tissues. It would appear, from their arrangement, that 
the intrinsic muscles are the contractor muscles of the heart, and that 
the extrinsic muscles function as dilators. 
^ Nordenskiolcl, E. (1909), p. 453. 
