54 Proceedings of the Koyal Society of Edinburgh. [Sess. 
to be directed postero-anteriorly in the case of the fifth space, and more 
medially in the anterior spaces till at the first space the direction was 
almost latero-medial — see figs. 1 and 6. (It is convenient for descriptive 
purposes to describe the inter-coxal space and the corresponding inter- 
branchial inlet together as the “ sub-branchial inlet.”) 
It is now possible to explain the meaning of “sand eddies.” When 
reversal occurred the exhalent stream was forced out through the sub- 
branchial inlets, the portion passing through the first appearing as the 
anterior eddy, while that passing through the remaining inlets as the three 
posterior eddies. 
During the normal forward action of the scaphognathites the converse 
Fig. 5. — Dorsal surface of Carcinus with a large part 
of the carapace and various organs removed. 
Semi-diagrammatic. 
Sc., Scaphognathite ; ls£ /., flabellum of 
first maxillipede ; H. = heart. 
Fig. 6. — Left gills viewed from the 
dorsum. Semi-diagrammatic. 
1-5, the arrows show the 
directions of the sub-branchial 
inlets. Note radial arrange- 
ment of the gills. 
to the above holds true. Water may then be inhaled from behind and 
from the sides of the animal through the channels just described into the 
branchial chambers. The latter are somewhat triangular in transverse 
and crescentic in oblique antero-posterior sections. 
The gills arise from an area which is roughly a segment of a circle — 
see figs. 5 and 6 ; so that, although they lie more or less latero-medially, 
their free medial ends almost meet. Their branchial septa occupy the 
plane above mentioned, while the lamellae are placed in the anterio-posterior 
plane. Lastly, there is a greater space above than below the gills. 
Of the pre-branchial chambers and their exits there is nothing of note. 
Conclusions. 
The normal direction of the respiratory current in decapods was shown 
to be postero-anterior by H. Milne Edwards (7). De Haan (6) at a later 
date remarked on the reversal of this direction in Portu^nus, though still 
