xxxiv 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 branchiae 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 



Fig. 11. 



in patches of irregular form, but Avhich 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, 



