66 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



three lobes, one anterior, one lateral, and one posterior ; 

 and each lobe has its main duct, into which all the tubes 

 composing it open. The three ducts unite together into 

 a wide common duct (bd), which opens, just be-hind the py- 

 loric valves, into the floor of the mid-gut. Hence the aper- 

 tures of the two hejMtic ducts are seen, one on each side, 

 in this part of the alimentary canal when it is laid open iJ 

 from above. Every caecum of the liver has a thin outer j 

 wall, lined internally by a layer of cells, constituting what 

 is termed an epithelium ; and, at the openings of the 

 hepatic ducts, this epithelium passes into a layer of some- 

 what similar structure, which lines the mid-gut, and is 

 continued through the rest of the alimentary canal, 

 beneath the cuticula. Hence the liver may be regarded 

 as a much divided side pouch of the mid-gut. 



The epithelium is made up of nucleated cells, which are 

 particles of simple living matter, or 'protoplasm, in the 

 midst of each of which is a rounded body, which is termed 

 the nucleus. It is these cells which are the seat of the 

 manufacturing process which results in the formation of 

 the secretion ; it is, as it were, their special business to 

 form that secretion. To this end they are constantly being 

 newly formed at the summits of the cseca. As they grow, 

 they pass down towards the duct and, at the same time, 

 separate into their interior certain special products, 

 among which globules of yellow fatty matter are very 

 conspicuous. When these products are fully formed, what 

 remains of the substance of the cells dissolves away, and 



