192 On the Circulatory System in the Decapod Crustacea. 



Carcinus mcenas) , and on land-crabs of the genus Cardtsoma, 

 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 bj definite investigations on 

 adults the rule enunciated by Glaus. 



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 with 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, tlie 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 Mysis, 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 lacunse 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. 



