()() THE PHYSIOLOGY OF THK 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 {h:l), which opens, just behind the pj'- 

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

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

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

 from above. Every cfecum of the liver has a thin outer 

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

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

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

 what similar structure, ^\'llich 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 nuich divided side pouch of the mid-gut. 



'i'he 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 teruied 

 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 caeca. 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, wluit 

 remains of the substance of the cells dissolves awav, and 



