88 THE FRESHWaTEE MUSSEL 



4, The structure of the gills. 



Gut the gills across, and examine the cut edges with a 

 pocket lens. Separate the two lamellm from each other ; and 

 mount small pieces of them and examine with the microscope. 



For a more minute study of the structure of the gills 

 microscopical sections should he made through various parts. 



The individual bars are slender, vertical, laterally 

 compressed rods, the inner edges of v?hich, i.e. those 

 turned towards the space between the two lamellsB 

 of the gill, are in most cases expanded laterally, and 

 fused with their neighbours. 



The bars are clothed with a single layer of 

 ciliated epithelial cells, which are cubical over the 

 greater part of the surface, but columnar along the 

 outer edge, a row of cells along each side of this 

 outer edge having peculiarly long cilia. Each bar 

 is strengthened by a double chitinous rod, which lies 

 close to its outer border. 



The inner or deeper parts of the bars consist 

 chiefly of lacunar tissue, i.e. a loose network of 

 branched cells, the meshes of which contain blood. 

 Distinct blood-vessels are comparatively few in 

 number ; they are of large size, and lie principally 

 in the interlameUar rods which bolt together the two 

 lamella of each gill. 



C. The Circulatory System. 



1. The pericardial cavity is an elongated space of con- 

 siderable size lying along the dorsal surface of the 

 animal, ventral to the ligament, and above the bases 

 of the gills. It contains the heart, and is traversed 

 by the rectum. Its walls are thin and semitrans- 

 parent behind ; thicker and spongy in front. Through 

 them the pulsations of the heart can be seen. 



Open the pericardial cavity by a longitudinal incision 

 along the right side, and cut away as much of its walls as is 

 necessary to expose the heart fully. 



