376 AEACHNIDA. 



retractor muscles of the chamber) to its posterior angle, and pass 

 backwards, leaving between the two sets of muscles a passage for the 



(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 veiy 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 to- 

 wards 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 Myiiapoda, 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 branchiaB, 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 supra- 

 spinal artery, forms the commencement of the sub -spinal vessel ; and 



