BRANCHIA. G3 



Branchia. 



In the Balaninse, a pair of Branchiae is always present : 

 they lie on each side, in a somewhat curved position, in the 

 angle between the sides of the shell and the basis. In 

 PI. 25, fig. 1, they are exactly covered, on the further side, 

 by the body of the animal. They are attached near each 

 other at the carinal end of the sack in a vertical line, and like- 

 wise on each side in a transverse line, extending from close 

 beneath the spur of the tergum towards the point of attach- 

 ment of the body to the scutum. In Balanus, as in the figure 

 (PI. 25, fig. 3) of B. tintinnabulum, each branchia consists of 

 a medial fold of skin, a little curved conformably with the 

 sack, and slightly tapering towards its rostral and free ex- 

 tremity ; but this fold is almost hidden by the vertical sub- 

 folds or membranous ridges, themselves plicated and sub- 

 plicated, which project on both sides : these vertical folds 

 are free at their tips : at their lower attached ends, they 

 are thickest. On the side nearest the wall of the shell, the 

 whole branchia has a bilobed appearance, owing to a very 

 deep indentation caused by the projection of the scutal 

 lateral depressor muscle ; the sub -folds on this side are also 

 more plicated. The branchia essentially is an inward 

 plicated fold of the membranes of the sack • for its outer, very 

 thin tunic is continuous with and moulted with that lining 

 the sack ; and within it we have two layers of delicate, pulpy, 

 transparent corium, united together (as is best seen in 

 Coronula) by ligamentous fibres, branched at their two ends, 

 all exactly as in the corium surrounding the sack. There 

 are here no distinct vessels, any more than in other parts of 

 the body, but a fluid could easily circulate in the interspaces 

 of the corium. From the large size of this organ, and its 

 simplicity of internal structure, being adapted exclusively to 

 expose a great surface of skin to the water, I do not doubt 

 that it has been correctly considered as a respiratory organ. 

 By the voluntary movements of the opercular valves {i. e. 

 part of the carapace) the water is constantly being pumped 

 in and out of the sack ; the movement, indeed, may be 



