THE CIECULATOEY SYSTEM. 93 



and then through the meshes of the gills into the 

 supra -branchial passages. It then passes backwards 

 along the supra-branchial passages to the cloacal 

 cavity, and so out. 



Respiration is probably effected mainly by the 

 inner surfaces of the mantle-lobes, and not by the 

 gills. 



4. The structure of the gills. 



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

 pocket lens. Separate the two lamellcs 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 filaments are slender vertical 

 laterally compressed rods, the inner edges of which, 

 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 filaments 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 fila- 

 ment is strengthened by a double cliitinous 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 num- 

 ber ; they are large, and lie principally in the inter- 

 lamellar rods which bolt together the two lamellae 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 



