308 Dr. T. Williams on the Mechanism of Aquatic 



longer proceed. Thus the Crustacean dies rapidly in perfectly 

 dry air ; but if, in the branchial chamber, a sufficient proportion 

 of humid vapour exist, the gaseous oxygen of the air (when the 

 animal is out of water) is dissolved, and the breathing proceeds as 

 efficiently as if the animal were still revelling in its native ele- 

 ment. This mechanism explains the capacity with which many 

 Crustacea are gifted, of living almost as well in air as in water. 

 This apparent amphibious power does not prove that when in air 

 they really breathe on the atmospheric plan, but that the oxygeu 

 of the air which is admitted into the branchial chamber is fluidi- 

 fied by the moisture which still remains in this cavity. The 

 mechanism of the respiratory process, even under such circum- 

 stances, is really aquatic in type. 



In ultimate structure the branchial laminae in the gills of the 

 Hermit Crab (PI. XVII. fig. 7, B), conform exactly with those of 

 the Brachyurous orders, subsequently to be explained. The 

 coats of the axial branchial vessel in the Hermit Crab are smooth, 

 and destitute of those hooks which are distributed so systemati- 

 cally over those of the common edible Crab (PI. XVIII. fig. 6). 

 The hooks are not required because there are no flabella. The 

 setse (PI. XVII. fig. 7, M) which depend from the roof of the 

 branchial cavity of the former present the mop-like character 

 of those already described in the Lobster. 



In the Brachyurous orders, exemplified by the vulgar edible 

 Crab, the gills are constructed on the leafy or laminar principle 

 (PI. XVII. fig. 9, A). They stand vertically in the containing 

 chamber. The leaves on each gill are arranged in two series {a), 

 one on either side of the longitudinal blood-channels. The shape 

 of the ultimate leaves diffiers in different species. In some they 

 are semilunar ; in others they are reniform, in others pointed, &c.; 

 in all they leave deep grooves for the free play of the branchial 

 current from the root to the apex of the gill, and from the apex 

 to the root on the other side. The streams passing horizontally 

 between the leaflets connect the two main longitudinal currents. 

 Thus the water- currents on the outside of the branchiae obsen^e 

 the same distribution and division as the blood in the interior. 

 The structure of the branchial laminse admits of resolution to 

 the very last elements of organic structure. They are incon- 

 ceivably slender and delicate. The most cautious attempt to 

 inject the gills converts the leaf into a bag, the lamina into a sac, 

 so delicately are the plates held together ; but though thus atte- 

 nuated in the extremest degree, each lamina consists of two 

 opposed plates. 



These plates are composed exclusively of a single layer of 

 epithelial cells, hexagonal in outline, and adjusted edge to edge. 

 Under the highest power of the microscope they exhibit minute 



