L. E. Eobinson and J. Davidson 
239 
Argas jiersicus are relatively shorter than those of the Ixodidae, a fact 
which is doubtless explained by the different habits of feeding. 
In their histological structure, excepting one or two details, the 
Malpighian tubules closely conform to Nordenskiold’s^ description of 
those of Ixodes ricimis. Each tubule is formed entirely of large 
secretory cells, the broad flattened bases of which rest on a delicate 
basement membrane. The free surfaces of the secretory cells are 
generally rounded and bulge out, moi’e or less, into the lumen of the 
tubule, the latter thus appearing, in transverse sections of the tubule, 
to send narrow diverticula between adjacent cells. The cytoplasm of 
the secretory cells is finely granular, but shows a very definite reticulum 
upon which innumerable secretion-granules are scattered. The nuclei, 
of moderately large size, are usually ovoid in shape and are situated near 
the bases of the cells. The chromatin is distributed in sharply defined 
angular masses, and each nucleus contains one or more nucleoli. Dis¬ 
solution of the nuclear membrane may occasionally be observed, in 
which case the nucleoli and small rounded masses of chromatin are 
found lying free in the cytoplasm. 
The free margins of the secretory cells generally show a narrow zone 
of homogeneous cytoplasm from which the reticulate structure is absent, 
but which sometimes exhibits a faint striation, the parallel striae run¬ 
ning in a direction at right angles to the free surface of the cell (the 
Stdbchensaum of Nordenskibld^). 
The secretion of the Malpighian tubules consists of a clear colourless 
fluid in which are suspended the singular, highly-refractive concretions 
which are so familiar to all who have examined microscopical preparations 
of ticks. These concretions are found scattered at intervals alonw the 
o 
whole course of the tubules, but in the rectal sac they appear in enormous 
numbers, and ultimately the sac is absolutely packed with them. They 
are never actually found within the secretory cells, and evidently crys¬ 
tallise out from the fluid secretion which the cells pour out into the 
lumen of the tubule. The Malpighian concretions, which appear 
yellowish by transmitted light, each show a very definite concentric 
striation, and in many cases, fine radiating fissures extend from the 
dark point, which forms the centre of the concretion, towards the 
periphery. Double concretions, in which the laminar deposits appear 
to have taken place round two adjacent nuclei, are common. 
As to the chemical nature of these concretions, it was supposed for 
■ Norclenskiokl, E. (1908), pp. 655-656. 
2 Nordenskiold, E. (1908), p. 655, and PJ. 27, fig. 14. 
