226 Argas persicns 
chitinoas layer. In longitudinal sections of the salivary duct, the spiral 
thickening certainly is seen to lie on the epithelial surface of the 
chitinous lining, but it is extremely difficult to decide that it is a 
separate structure. Nordenskiold^ has observed that in torn sections, 
where the salivary duct is ruptured, the spiral thread comes away with 
the cell-protoplasm and not with the chitinous cuticle, as is the case 
with a trachea. On the other hand, we have noticed that specimens 
which have been cleared in potash, and in which, therefore, all but the 
chitinous structures are dissolved, the spiral thickening of the salivary 
duct is still clearly visible on the outer surface of the chitinous lining. 
Nordenskibld suggests that the spiral thickening is a contractile thread 
which functions as a regulator of the salivary duct, but, in the absence 
of further evidence, we are inclined to consider the structure as a spiral 
thickening of the chitinous cuticle which projects on the epithelial 
surface and which serves to maintain the patency of the ducts. 
As already mentioned, the cuticular lining of the salivary ducts 
extends quite to the mouths of the alveoli, and, in the case of the 
alveoli of the first type, just at its termination within the mouth of each 
alveolus, the lining is slightly thickened and frequently shows a few 
longitudinal folds. This is, without doubt, the appearance which 
Nordenskibld interpreted as a valvular structured 
The main salivary ducts become free of the glands just before they 
pass through the capitular foramen (see PI. XIV,/!c.). Within the basis 
capituli they converge slightly as they run forwards beneath the lateral 
margins of the subcheliceral plate towards the buccal cavity, into the 
postero-lateral angles of which they open. (Part I, PI. V, figs. 18-20, 
and PI. VI, fig. 21, d. sal.) 
Reference must now be made to the histological structure of the two 
types of alveoli which have been previously alluded to. Bonnet- was 
apparently the first to recognise the two forms as they occur in I. ricinus, 
and Samson^ subsequently confirmed his observation ; Nordenskibld, 
however, makes no mention of any distinction although one of his 
figures'* gives convincing evidence of their existence. 
The greater mass of the gland is formed of alveoli of what, up to 
the present, have been referred to as the first tyjie, and it can only be to 
this type that Nordenskibld’s very detailed description applies. 
Each alveolus of this first type (PI. XVII, fig. 5) is roughly pear- 
shaped, the smaller end being the place of attachment to the efferent 
1 NordenskiOld, E. (1908), p. 649. = Bonnet, A. (1907), pp. 51-58. 
3 Samson, K. (1909 «), pp. 202-205. ■* NordenskiOld, E. (1908), PI. 27, fig. 12. 
