OF MYRIAPODA AND MACROUROUS ARACHN1DA. 
295 
spinal vessel becomes very much smaller, and about midway between the second and 
third abdominal ganglia sends off a pair of smaller branches, which are given in like 
manner to the posterior part of the same branchiae. It then continues its course as 
a very minute trunk to the third ganglion. At this point the direction of the lateral 
vessels given off from it is altered (tig. 33. n. 1). Instead of passing laterally backwards 
they are now directed outwards and forwards, to distribute the blood to the anterior 
part of the fourth pair of branchiae. This altered direction of the vessels is necessary 
in order to convey inwards to the body the blood that is passing forwards along the 
subspinal vessel in the tail to be aerated in the posterior branchiae of the abdomen 
before it is again transmitted to the heart. That this is the case is proved by the 
fact, that a single median trunk ( n . 2.) is given off from the under surface of the sub- 
spinal vessel beneath the ganglion in the first caudal segment. This trunk is directed 
forwards into the seventh or terminal segment of the abdomen, lying loosely like 
those which are directed backwards in the anterior segments. When opposite to the 
posterior part of the fourth pair of abdominal branchiae it is dilated and divided into 
two branches, which together form a dilatation like the anterior ones before they are 
directed outwards and are joined by other venous trunks and distributed over the 
branchiae. This is the mode of distribution of these vessels in the great Scorpion, 
Buthus afer, Leach, but a slight variation is found in some other species. Thus in 
the specimen formerly* examined, Buthus costimanus, Koch, or a species nearly allied 
to it, instead of a pair of branches given off from the subspinal vessel midway between 
the second and third abdominal ganglia, I found an azygos vessel given off beneath 
the latter, and which having passed forwards, midway between these ganglia, was then 
divided into lateral branches like those at the anterior part of the body. 
This peculiar distribution of the subspinal vessel in the abdomen enables us to un- 
derstand a fact that seems at first very difficult of explanation ; viz. that the branches 
from the caudal artery on the dorsal surface of the tail do not anastomose with the 
spinal artery (/’) that lies above the cord, and in which the course of the blood, as we 
have already seen, is from before backwards, but with the subspinal vessel beneath it 
(w),in which the course of the blood is inwards to the abdomen, precisely that of the 
dorsal artery; so that the unemployed blood from this structure, and that which has 
become venous, is collected as it returns from the tissues by the subspinal vessel, and 
intermingled together before it is transmitted to the branchiae. 
Structure of the Pulmono-branchicc . — Professor Muller^ lias already accurately de- 
scribed the pulmono-branchiae as formed of a multitude of closely approximated, 
thin, double lamellae, which communicate by a small orifice in each with the ex- 
ternal air admitted into a common cavity through the spiracle on the surface of the 
body. The blood distributed through these lamellae is brought into contact with the 
air in their interior through their membranous structure. The minute anatomy of 
these lamellae, and the manner in which they are permeated by the blood, afford some 
* Loc. cit. f Meckel’s Archiv, 1828. 
