Affinities of Pteronarcys regalis. 431 
of the mesothorax, two filaments sometimes arise from a common origin, in 
which case they are supplied with their tracheze from the same root (c), but 
these are exceptions to the general structure. "The manner in which the sacs 
are supplied with tracheze directly from the great trunks of the body, and the 
distribution of branches of these to the filaments, are important considerations, 
as the demonstration of these facts fully proves that the sacs in the imago Pte- 
ronarcys are true respiratory organs. With this object I removed the anterior 
mesosternal sac from the left side of Pteronarcys in connexion with a portion of 
tegument and of trachea (fig. 3 a), and found on examination beneath the mi- 
croscope that this sac is supplied by a large short branch from the great trunk 
that passes across the mesosternal surface from the bundle of trachez poste- 
rior to the prothoracic spiracle, from which bundle other trachez pass to the 
anterior pair of wings, to the cesophagus, and to the dorsal muscles. _ A pre- 
cisely similar mode of distribution exists also in the metathorax (fig. 10 i, &). 
The division of the trachea within the sac differs a little from what has been 
described and delineated by M. Pictet * in the larva of Perla. In that spe- 
cies the tracheze are shown to divide abruptly into a multitude of minute 
ramifications. In Pteronarcys the trachea (fig. 4 b) divides immediately it 
enters the root of the branchia into two branches. Each division soon again 
separates into two, and these again each into two others, and this binary mode 
of distribution is repeated until the whole terminate in ramifications of simi- 
lar diameter, one of which enters each branchial filament (fig. 4 c), and, gra- 
dually becoming smaller from its base to its termination, gives off other small 
ramifications as it passes onwards, and terminates by dividing into a pair of 
. exceedingly delicate closed tubes. M. Pictet remarks that he has not been 
able to satisfy himself in what way the ramifications terminate, and thinks 
that the interior of these is continuous at their apex with the * muqueuse in- 
terne du tube,” the filament. Iam not able to confirm this opinion. On the 
contrary, I have reason to believe that the terminations, becoming gradually 
more and more delicate, end as ceca. . i 
Circulation of the Blood.—The blood-corpuscles of the whole body circu- 
late through the branchize for the purpose of aération. "i current of blood 
is always in the vicinity of tracheal vessels, “ whether simply along inter- 
bid. p. 88. 
* Loc. cit. p. 87. pl. 3. figs. 3 & 4. T Ibid. p 
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