XXX1V INTRODUCTION. 
character and appearance of an internal skeleton. The 
branchial organs are covered and protected, but they are, 
nevertheless, essentially external appendages. In the 
Amphipoda this condition does not exist; consequently 
the branchiz are pendant in the water, and placed on the 
inside of the pereiopoda, the first joints of which are 
developed into large squaminiform plates for their more 
efficient protection. 
The internal structure of these organs appears to consist 
of thick fibrous tissue attached to the inner surface of the 
wall of each sac (Fig. 11). The fibrous tissue is arranged 



















































in patches of irregular form, but which correspond in 
their arrangement with one another. These patches are 
largest near their centre, and thin out towards their mar- 
gins: the result is that a channel is left between each. All 
the channels so formed are connected together throughout 
the entire organ, and exhibit a continuous labyrinth, 
through which the blood circulates in many small 
streams. 
Should the animal become feeble, a gradual accumula- 
tion of corpuscles takes place in different parts of the gills, 
