57 



tcistine is largest in front and tapers posteriorly to the very 

 narrow hinder part which is called the rectum. That portion 

 of the large intestine in front of the point of entrance of the 

 small intestine may be called the intestinal ccecum. 



For an account of the functions of the different parts of 

 the alimentary canal see the Cambridge Natural History, 

 Vol. v., pp. 123-127. 



Salivary glands. — The salivary glands lie one on each 

 side of the oesophagus. Each is a white, firm U-shaped body, 

 with the two arms much thickened, and the inner reaching 

 a little farther forward than the outer. The salivary duct 

 arises from the outer lobe ; the two ducts run forward and 

 unite beneath the oesophagus, the common duct thus formed 

 opening into the mouth-cavity. 



Without removing the alimentary canal, make a drawing 

 of it and the salivary glands in position in the body. 



Examine a piece of salivary duct in water or glycerine on 

 a slide under the microscope. Note the transversely striated 

 condition. Remove the cover-glass and with needles pull 

 the duct gently apart. Examine again ; the two parts will 

 probably be connected by a spiral thread ; this will be 

 seen to be what formed the transverse striation ; it is really 

 a spiral thickening of the walls, not in a continuous thread, 

 but in a series of short, independent spirals of one or few 

 turns, called tcenidia. 



The respiratory system. — The external openings, spir- 

 acles, of the respiratory organs, have already been noted in 

 the examination of the external parts of the specimen. Each 

 of these two spiracles opens internally into a main air tube, 

 trachea, which runs forward through the whole length of the 

 body. .In each of the segments, from the fourth to the tenth 

 inclusive (not counting the head), a large branch trachea is 

 given off from the main longitudinal trunk to the alimentary 

 canal, and a smaller one to the dorsal blood-vessel (see J>ostea). 



