In addition to the above arrangement, Mr. Newport has discovered a fibrous structure, from which are given off two pairs of vessels, to be distributed to the first pair of branchial organs, as also a little vessel which, passing backwards, anastomoses with the spinal artery, to form the sub-spinal vessel. This latter takes its course beneath the chain of nervous ganglia, communicates directly by means of short branches with the supra-ganglionic artery, and, at intervals, gives off from its under surface large vessels, which, uniting together, convey the blood which has circulated in the abdominal segments directly to the branchiae, whence it is returned to the heart by a great number of slender canals, which, emanating from the posterior aspect of each branchial organ, unite to form larger trunks, that run along the walls of the segments, to pour their contents into the valvular orifices situated upon the dorsal aspect of the heart.

* Audouin, Cyclop. of Anat. and Phys., art. "Arachnida".

(966). The heart of the Scorpion* is a strong muscular organ extending along the middle of the back, from its continuation with the great caudal artery in the last segment of the abdomen to the commencement of the aorta. 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 Myriapoda. 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 necessarily propelled. The form of each chamber 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.

(967). Each chamber is provided at its sides, as in the Myriapoda and Insects, with two sets of muscles, the alae cordis. The anterior and larger 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 pass backwards, leaving between the two sets of muscles a passage for the vessels.

* Newport, Phil. Trans. 1843.

(968). The structure of the chambers internally differs considerably from that of the chambers in the Melolontha, as described by Straus-Durckheim (§ 852.) Each valve, or division between them, is formed by a reduplication of the whole muscular structure of the dorsal surface of the organ. This reduplication, 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. The reason for this imperfect structure of the valves may perhaps be explained by the fact that the blood is distributed from the heart, in the Scorpion, in opposite directions - partly backwards towards the tail, but chiefly forwards towards the head and sides; and hence it may be necessary that a reflux of the blood should not be entirely prevented, as may be required in those instances in which the whole current is in one direction. The structure of the heart is exceedingly thick, opake, and muscular; it is formed of two layers of fibres, longitudinal and circular in each layer, the most powerful of which are the latter. On its internal surface it is smooth, and lined by an exceedingly delicate membrane, through which the strong circular fibres are distinctly marked.

It is by means of these that its most powerful contractions are effected, - the auricular action being chiefly the result of the relaxation of these fibres, assisted by the reactions of the lateral muscles.

(969). The aorta arises from the anterior extremity of the heart or dorsal vessel. It is short, thick, and smooth on its external surface, without lateral muscles or internal divisions into chambers. It descends obliquely forwards and downwards, and after passing beyond the great median arch of the thorax, to which many of the muscles of this region of the body are attached, it gives off the vessels to the head, to the organs of locomotion, and to form the great supra-spinal artery, which, as in the Myriapoda, represents the aortic trunk, or rather the aorta descendens, which, running above the chain of nervous ganglia, supplies the neighbouring parts in this region of the body, as well as branches to the alimentary canal and to the liver.

(970). The portal system of vessels is situated chiefly below the nervous cord, on the ventral surface of the body, and is the means by which the blood is collected and conveyed to the branchiae, from which it seems to be returned to the system, after circulating through the organs, by means of a large sinus or vessel at their posterior superior angles. Behind the bony arch of the thorax there is a hollow fibrous structure, that closely surrounds the cord and nerves as in a sheath. It seems to form a kind of sinus, from the posterior part of which a small vessel passes backwards, which, joined by anastomoses from the supraspinal artery, forms the commencement of the sub-spinal vessel; and it gives off two pairs of vessels at its sides. The first and second pairs of these efferent vessels, covered by the thick peritoneal lining of the abdomen, send the blood in a diagonal direction backwards to the first pair of abdominal branchiae. The first pair of these vessels originate close to the folds of the diaphragm.

They pass backwards and outwards into the abdomen, and are joined in their course by numerous small vessels from the sides of the segments, and immediately anterior to the first pair of abdominal branchiae are each divided into two branches, which are again divided and subdivided into a multitude of anastomosing vessels before they are distributed on the branchiae. These branchiae likewise receive the second pair of efferent vessels, which, like the first, pass diagonally backwards from the fibrous structure to the inner side of the branchiae, on approaching which they are divided like the other pair into two branches, which are subdivided, and anastomose with the divisions of the first pair. The whole form a most intricate web of anastomosing pulmonic capillary vessels before they are distributed on the anterior part of the branchiae. We have thus a complete distribution of the blood to the pulmonibranchiae in the anterior part of the abdomen. There is a similar but less perfect distribution in the posterior.