350 
The Spiracles of a Tick 
broken at intervals by the external pores ( e.p .) and directly beneath 
the latter are seen the internal pores (ip.) passing through the basal 
layer. Large pyriform air spaces are formed between the groups of 
pedicels beneath each of the external pores, and these pyriform spaces 
( p.s .) establish direct communication between the two series of external 
and internal pores. 
The portion of the spiracle beneath the macula shows an entirely 
different structure. It is occupied by a columnar mass of connective 
tissue and muscle fibres running upwards from the soft structures 
underlying the spiracle; this mass of soft tissue, which we have termed 
the columella, extends up to the under surface of the superficial layer 
which is considerably thickened at this central portion (PI. XXIII, 
figs. 7—10 col.). Completely encircling the columella is a large annular 
air space, the pericolumellar space ( pc.s.) which communicates directly 
with the interstices between the pedicels in the intermediate space. 
The ostium opens into the pericolumellar space on the internal side 
of the columella (PI. XXIII, figs. 5 —7) and immediately beneath the 
ostium the cavity of the pericolumellar space is continued downwards 
as a large chamber of irregular shape which we have termed the 
atrium (a.). The cavity of the atrium is roughly elliptical in cross 
section, somewhat contracted dorso-ventrally; its direction is at first 
horizontal or slightly upwards, it then bends downwards and terminates 
abruptly, the entire length being equal to about half the diameter 
of the spiracle. Numerous main tracheal trunks (tr.) open separately 
from the basal portion of the atrium, running inwards and downwards 
for a short distance as a stout bundle of tubes with spirally thickened 
walls, from which the tracheae radiate to all parts of the body. 
Running up the columella and inserted along the dorsal wall of the 
atrium (to the right of the atrium in the figures on PI. XXIII) is 
a band of muscle fibres, a small portion of which runs up to the 
superficial layer and is inserted beneath the centre of the macula. 
The contraction of this columellar muscle would dilate the cavity of 
the atrium and possibly at the same time close the ostium, the external 
margin of which overhangs the slit-like opening. This would cause 
the inspired air to filter through the external pores of the superficial 
layer. The expulsion of air from the tracheal system is effected by 
the contraction of the dorso-ventral body muscles which squeezes the 
contained air out through the spiracle, a fresh supply being inspired 
by the elastic rebound of the tracheal tubes, when the action of the 
body muscles is relaxed. 
