192 On the Circulatory System in the Decapod Crustacea . 
Carcinus mcenas ) , and on land-crabs of the genus Cardisoma, 
have enabled me to study in all its details the circulation in 
the membranous walls which clothe the carapace in the bran- 
chial region, and to substantiate by definite investigations on 
adults the rule enunciated by Claus. 
The afferent system of the membrane which clothes the 
carapace in the branchial regions has its origin in the vast 
postcephalic lacuna which surrounds the liver and the entire 
stomach ; a quantity of blood, very variable in amount in 
the different types, also enters this membrane by the ultimate 
branches of the lateral anterior (antennary) and posterior 
arteries. In the land-crabs of the genus Cardisoma , as in 
Birgus latro, which was studied by Semper, the largest por- 
tion of the blood is drawn from the ventral region of this 
lacuna and forms a large trunk in front, which then divides 
into several branches, the secondary divisions of which are 
very numerous, very minute, and gather themselves into a 
plexus ; but in the more distinctly aquatic Decapod Crustacea 
the large afferent trunk usually does not exist, and we are 
confronted wdth an infinite number of little anastomosing 
lacunar canals, which detach themselves from the lacuna at 
its points of contact with the membrane. 
The efferent system is absolutely constant ; it consists of a 
well-defined trunk which follows the membrane close to the 
lower free border of the carapace ; very narrow anteriorly, 
this trunk receives on its way the efferent branches of a plexus 
which is continuous with the afferent plexus ; it increases 
considerably in size the further back it gets, and opens 
directly into the pericardium either at its posterior angle 
( Astacus ) or at the sides (edible crab, Cardisoma). The 
whole of the efferent system, the pericardium, and the entire 
arterial system can be easily injected by way of this large 
efferent trunk. 
In studying the disposition of the afferent and efferent 
canals in this region of the membrane we are soon convinced 
that we are dealing with a cutaneous respiratory apparatus 
analogous to that of My sis, and that it is the exaggeration of 
this arrangement which allows certain Crustaceans (land- 
crabs, Birgus latro) to live a very long time out of the water. 
In other words, the blood which returns directly to the peri- 
cardium by way of the large efferent trunk of the carapace is 
not venous but arterial blood. It is possible that a portion of 
the venous blood of the lapunge returns directly to the peri- 
cardium, and we even find two orifices at the bottom of the 
pericardial sinus of the edible crab, which seem to be intended 
to serve this purpose ; but in any case we are bound to con- 
cede to the system of the carapace an efficient respiratory role . 
