51 
1917-18.] Respiratory Mechanism of the Shore-Crab. 
escapes. The base of this triangle corresponds to the breadth of the 
inter-orbital projection. Experiments V. and VI. were repeated, with 
similar results. 
Exp. VII. — Ink was injected into the sand half an inch behind and to 
the side of the animal about its own level beneath the surface, with the 
same result as Experiment VI. 
Reversion currents were also observed, and were found to comply with 
the description in the last experiment. 
Exp. VIII. — I covered some crabs completely with sand to a depth of 
an inch, and almost immediately the sand over the prostomium was seen 
to be thrown up by the exhalent current. The opening formed was 
Fig. 1 . — Ventral surface of Carcinus, legs removed at coxo-basal joints. 
Semi-diagrammatic. 
Ex. , exhalent stream; IN., 1-4, inhalent streams; Ant., anterior 
“sand eddy” ; Post., posterior “sand eddies.” Note the direction of 
the individual inhalent streams, which is also that of the “sub- 
brancliial inlets.” 
gradually enlarged until the triangular communication mentioned above 
was attained. Experiments V., VI., and VII. were repeated, with the same 
results. One small crab, however, remained buried without forming the 
communication — the exhalent current being simply forced through the sand. 
Channels of Respiration. 
! 
The observation noted in Experiment VI. regarding the occurrence of 
“ sand eddies ” led me to determine the manner of their origin and hence to 
closely inspect the respiratory channels. 
These are, briefly, the sub-branchial clefts, the branchial and pre- 
branchial chambers, and the prostomium : the inhalent apertures being 
the sub-branchial clefts, and the exhalent, the opening of the pre- 
branchial chambers into the prostomial region. 
Each cleft is merely a slit, bounded medially by the base of the thorax, 
