L. E. Robinson and J. Davidson 
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gland. The outer wall of the duct consists of an epithelial layer, the 
individual cells of which are not defined, but the cellular nature of which 
is evident from the presence of numerous small ovoid nuclei which are 
embedded in the pale, very finely granular protoplasm. The most 
remarkable feature of the salivary ducts is the spiral thickening of the 
inner portion of the duct wall. The entire lumen of all the ducts 
possesses a chitinous lining continuous with the cuticle of the buccal 
cavity, and which extends to the very mouths of the. gland alveoli. 
This cuticular lining shows a spiral thread-like thickening, an appeai’ance 
Fig. 1. Argas persicus $ . A. Transverse section of the anterior half of the right 
salivary gland, showing the relative positions of the two types of alveoli, x 320 diam. 
B. The entire gland from the dorsal aspect. The shaded area indicates the situation 
of the alveoli of the second type —the ‘ ‘ Pyramideuzellen ” of Samson. 
which has led the majority of observers who have described the structure, 
to compare it with that of the typically spirally-thickened air-tubes of 
tracheate Arthropods. Nordenskiiild\ howevei-, has objected to such a 
comparison, on the ground that the spiral thickening of the .salivary 
ducts is not a part of the cuticular lining, but is actually embedded in 
the protoplasmic epithelium and, therefoie, lies entirel}' without the 
' Nordenskiold, E. (1908), pp. 648-649. 
