OF MYRIAPODA AND MACROUROUS ARACHNIDA. 
287 
cephalothorax. The aorta descends obliquely forwards and downwards, between 
the muscles, to the oesophagus, on which it is spread out, behind the brain, into 
several large vessels, the two posterior of which are united beneath the oesophagus, 
and give origin to the great spinal artery (i ) — the structure above noticed. In the 
dorsal part of its course the heart is divided into eight separate chambers, which 
are wider and stronger in proportion to their length than in the highest of the My- 
riapoda — the Scutigeridce. They are more muscular and compact in proportion to 
the greater quantity of blood to be transmitted through them, and the force with 
which it is necessary to be propelled. The form of each chamber (PI. XV. fig. 34.) is 
somewhat heart-shaped, being slightly contracted in its middle portion, and enlarged 
at its posterior. Each chamber has two auricular openings («) for the passage of the 
blood, placed very close to the median line of the heart on its dorsal surface ; and it 
gives off at its inferior lateral angles a pair of large arterial vessels (/?), the systemic 
arteries, which distribute the blood downwards to the viscera, and to the dorsal and 
lateral surfaces of the body. These are the vessels imperfectly noticed by Treviranus. 
Each chamber is also provided at its sides, as in the Myriapoda and insects, with two 
sets of muscles, the alee cordis. The anterior and largest pair of muscles are attached 
to the anterior part of each segment, and pass diagonally forwards, and the posterior, 
the proper retractor muscles of the chamber, to its posterior angle, and are directed 
backwards, leaving between the two sets of muscles a passage for the vessels. Of the 
eight chambers that form the heart in the Scorpion, the posterior two are the smallest, 
and are situated in the seventh, or last segment of the abdomen. The eighth chamber 
is very imperfect, and is continuous with the caudal artery, which passes backwards 
to the post-abdomen, or tail ( q . q.). The sixth chamber is the largest and most 
powerful of the whole organ, and seems to correspond to the two chambers in the 
nineteenth and twentieth segments of the Scolopendra, and which, in that animal 
also, are the largest. Each succeeding chamber, from the sixth to the anterior one 
(t), at the termination of the heart in the aorta, as it enters the thorax, is shorter, 
and narrower than the one next behind it, and the systemic arteries ( p p) are smaller 
than those of the posterior. The structure of the chambers internally differs consi- 
derably from that of the chambers in the Melolontha, as described by Straus Durck- 
heim. Each valve or division between them is formed by a reduplication (fig. 36. u) 
of the whole muscular structure of the dorsal surface of the organ. This reduplica- 
tion, which is chiefly on the upper and lateral surfaces, is very imperfect on the under, 
and in some of the chambers is entirely absent on the under surface. It is extended 
about midway into the interior, which it divides more completely than in its less per- 
fect state of development in the young Myriapoda. It is most complete in the sixth 
abdominal segment, between the largest chambers of the heart. In the middle ones 
it is extended inwards and forwards, in the form of a broad nipple-shaped protube- 
rance, the orifice of which is opened behind in the middle line. This protuberance 
(fig. 37.) is situated above, and a little behind, the two large outlets ot the systemic 
