8 Stewart, Branchial Lamellae of Ligia oceanica. 



It is interesting to notice that Delage (2) in his 

 description of the circulation of the blood of Ligia, states 

 that the blood arrives from the venous cavities of the 

 body into the internal horde)' of the gill and returns round 

 the external border to the pericardium, after traversing a 

 system of lacunae. 



It follows therefore that the external pressure on the 

 gill surface would be greatest at the point where oxygen 

 is being drawn into the impure blood : the fact that dis- 

 tortion of the gill lamella, after immersion in fresh water, 

 invariably takes place on the internal border would seem 

 to lend support to this view. 



In the case of forms exposed to sea-water, there is no 

 such sudden change of osmotic pressure acting on the 

 most active point of the gill-surface, because the moisture 

 already contained in the gill-cavities is probably of a 

 saline composition ; the gradual change of shape, there- 

 fore, which occurs at every point on the outer surface, is 

 due more to the long-continued effect of the altered 

 pressure acting evenly on the gill-surface. 



These observations seem to point to the following 

 conclusions : — 



I. The internal lamella of the gill is more directly 

 concerned with respiratory functions than the 

 external lamella, because it is at once affected by 

 any changes in external conditions ; the fact that 

 the exopodite is not altered, even by a sudden 

 change, would appear to indicate that its res- 

 piratory functions, if they exist, are very slight. 



II. Continuous contact with water for any period of 

 time causes a corresponding change in the mode 

 of breathing, and hence in the gill structure, 

 which ultimately leads to death. 



