130 DE. L. A. BORRADAILB ON THE 
podobranch may thus have a greater physiological importance than its size 
would suggest, though it seems doubtful whether in its more isolated position 
the water would be drawn between its leaflets. The divarication of the third 
maxilliped also exposes an opening between it and the limb in front of it 
which helps to admit water to the fore part of the chamber. 
2'lie course of the water ivithin the gill-chamber is more difficult to follow, 
but there can be no doubt that on entering, it passes under the gills, that is 
into the hypobranchial space, tlien comes outwards between the gill leaflets 
into the epibranchial s[iace, and finally flows by way of the collecting-space 
into the exhalent passage. That practically all the water which enters the 
gill-chamber takes this course, and docs not pass directly from the exterior 
to the epibranchial space, I am convinced by the following considerations : — 
(1) It is exceedingly unlikely that the blood in the leaflets of the gills is 
exposed to the stream of water only on their edges, and that their flat 
surfaces are dependent on eddies or diflfusion for the renewal of the water in 
contact with them. (2) When carmine is caused to enter through any of 
the openings it is found principally, and the large particles are always found, 
underneath the gills. (3) The disposition of the parts is such as to suggest 
that the current; flows in the direction that I have described. From each 
of the openings behind that of Mihie-Edwards the shortest route to the 
exhalent passage leads" under and through the gills. If there were a wide 
space between the gills and the border of the branchiostegite, the resistance 
due to friction with the leaflets might cause the water to pass over the gillSj 
but actunlly the branchiostegite fits, as has been said, close against the gill- 
bases. In front of the cheliped the relations of the parts are different. 
Here, when the third maxilliped is in its normal position, its coxa and 
epipodite bar the passage of the water forwards and direct it inwards under 
t:he gills ; but when the maxilliped is divaricated, the shortest route to the 
inhalent channel would be, were it not for the barrier formed by the 
epipodite of the first maxilliped, directly forward. It would then pass partly 
through the podobranch of the second maxilliped, but in great part between 
that structure and the floor of the chamber or the bases of the other gills, and 
would not be distributed to the latter organs, which lie above the direct 
course. Actually, however, the existence of the barrier mentioned must 
prevent the water from being chiefly drawn in this direction and cause it to 
circulate through the upper gills. That the water does actually take the 
route through the gills can easily be seen by cutting a window in the floor 
of the exhalent passage and placing carmine in Milne-Edwards's opening. 
The carmine will be found always to pass under the gills and to reappear- 
above the scaphognathite, not to take the direct route. 
The water which enters above the first and second walking-legs flows 
forwards and inwards along a shallow gutter hollowed on the epimerite tillit 
reaches the entry of a ht/pobranchial channel between the pleurobranch of its 
