508 



LECTURE XXI. 



the bar ; i the septum, and g the interlaminnr canal. Each blade- 

 shaped bar supports three rows of cilia on each side of its base. 



iM 



% 



m 



190 



-V^-r 



■ " Branchial bar and cilia magnified. Mijtilns cdulis. 



Tlie minute particles suspended in the branchial currents are carried 

 by the ciliary actions towards the mouth ; and the water is filtered 

 through the interlaminar canals before it escapes. The walls of the 

 interlaminar tubes support a regular network of blood vessels, longi- 

 tudinal and transverse, the latter being most prominent : the meshes 

 are parallelograms, and form open spaces, fringed internally by a 

 narrow ciliated membrane. The cilia compel the requisite move- 

 ments of the water in the branchial chamber, when the bivalve 

 remains suspended in the air, as happens to a mussel attached to a 

 rock above low-water mark. Even when the animal is in such a 

 position as to be immersed only for about two hours in seventy-five 

 days out of the year, it can live and grow : the retained water de- 

 riving oxygen from the atmosphere, and the animalcular food propa- 

 gating therein. The life of the "tree-oysters" {Ostrea polymorpha) 

 suspended to the mangrove branches is similarly explained through 

 the wonderful mechanism of the ever active microscopic cilia. 



The two gills of one side are usually connected with those of the 

 opposite side by their hind ends only ; but somesimes the union is 

 more extensive. In a few genera, as Anatiria and Pliolodomya the 

 two gills of the same side are so united as to appear like a single 

 gill. In the Pholadomya this forms a thick oblong mass, finely 

 plicated transversely, attenuated at both extremities, slightly bifid at 

 the posterior one. A line traverses longitudinally the middle of the 

 external surface, which has no other trace of division. The branchia; 



