L. E. Robinson and J. Davidson 
249 
branches running to the middle line of the ventral body-wall, where 
they anastomose freely with their partners of the opposite side. 
(3) A large postero-dorsal trunk (tv. post, dors.) \vhich curves up¬ 
wards through the great notch between the antero-lateral and postero¬ 
lateral lobes of the stomach and then continues its course over the 
dorsal surface of the postero-lateral lobe. At the point where this trunk 
appears on the dorsal surface of the gut, a short branch arises which 
runs inwards and forms an anastomosis which surrounds the heart. 
The remainder of the branches of the postero-dorsal trunk are dis¬ 
tributed over the whole postero-dorsal region of the body. 
(4) A posiero-median trunk {tr. post, med) runs towards the median 
line and then backwards, to furnish branches to the inner half of the 
postero-ventral region of the body. 
(5) A postero-lateral trunk (tr. post, lat.) leads directly backwards 
from the atrium and divides into branches which ramify in the outer 
half of the postero-ventral part of the body. 
Variations from the courses described above are generally due to 
the fact that the smaller trunks such as the antero-median and the 
postero-median, instead of proceeding directly from the atrium, arise as 
branches from the proximal portions of the large anterior and postero- 
dorsal trunks, respectively. Such a case appears in the figure (Text- 
fig. 7), in which the antero-median trunk is seen to take its origin 
from the anterior trunk. 
The ultimate divisions of the tracheae are exceedingly fine, and as 
they ramify over the surfaces of the organs they may be seen to pene¬ 
trate between the cells. In I. ricinus Nordenskibld^ has observed that 
the tei-minal twigs of the tracheae actually enter the bodies of the cells ; 
he specifically mentions the secretory cells of the hypodermal glands, 
the nerve cells and, particularly, the contractile substance of the body 
muscles. 
The histological structure of the tracheae of the Ixodoidea has been 
described by most of those who have worked on the anatomy of ticks. 
Though there appears to be some doubt as to whether they are homo¬ 
genetic with the tracheae of the Antennata, they are apparently identical 
in structure. Each trachea possesses an inner cuticular sheath which is 
spirally thickened, and an outer epithelium of flattened cells containing 
a finely granular cytoplasm and flattened nuclei, the same structure 
extending from the largest tracheal trunks to the finest ultimate 
twigs. 
1 Nordenskiold, E. (1909), pp. 4.57-458, and PI. 30, figs. 4 and 5. 
Paragitology vi 
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