58 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



layer, composed of an internal sheet of circular, and external sheet 

 of longitudinal fibres. The muscle layer is thicker, and has a 

 more complicated arrangement in the pharynx, crop, and gizzard 

 than elsewhere. Outside the muscle there is a layer of delicate 

 flattened peritoneal epithelium. The " hepatic cells " are elon- 

 gated and fusiform, with well-marked nuclei, and granular proto- 

 plasm often containing concretions. They are closely connected 

 with the blood-vessels. 



Earthworms live principally on vegetable food, but animal sub- 

 stances can be used as well. Much earth is swallowed by them, 

 chiefly for the sake of the contained organic matter, but also as a 

 means of excavating burrows. Darwin has shown " that in many 

 parts of England a weight of more than ten tons of dry earth 

 annually passes through their bodies, and is brought to the 

 surface on each acre of land." Pieces of leaf, &c., are partially 

 digested before being taken into the body by a fluid poured on to 

 them from the mouth, and apparently secreted by the pharyngeal 

 glands. Food is taken in by alternate dilatation and contraction 

 of the mouth, aided by a sucking action of the pharynx, and 

 organic acids contained in it are neutralized by the carbonate of 

 lime secreted in the calciferous glands.* 



Food accumulates in the crop, and is then passed on to the 

 gizzard, where the contractions of that organ grind it up. This 

 process is largely helped by small stones which have been swal- 

 lowed and act as millstones. The contractions of the muscular 

 wall of the intestine cause the substances which enter it to be 

 passed slowly backwards. They are meanwhile subjected to -the 

 action of a digestive juice, similar to that poured from the mouth, 

 and probably secreted by the intestinal epithelium. This appa- 

 rently contains ferments^ which bring the starch and proteids 

 into a state of solution, the former being converted into grape- 

 sugar, the latter into peptones. Any fat swallowed is brought 

 into a very fine state of division i.e.. emulsified. The particular 

 ferments which bring about these changes can only work well 

 in an alkaline solution, hence the use of the eesophageal glands. 



* This lime may also be regarded as an excretion. A superabundance of 

 it is taken in with the food, and is thus got rid of. 



t Ferments are bodies which excite chemical changes in other bodies 

 without themselves entering into the reactions. They are (1) Living e.g., 

 Yeast and Bacteria; (2) Non-living e.g., digestive ferments, complex 

 nitrogenous bodies found in digestive fluids. 



