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ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 221 
median aorta and the paired common carotids lie morphologically outside the roof and 
floor of this sinus respectively. Below the gut is another large sinus of similar form, 
which covers practically the dorsal surface of the club-shaped muscle.* Laterally 
to the gut these two sacs are separated by a thick zone of fatty tissue, but, as it is 
easy to demonstrate by inflation and by injection, they are in frequent and wide 
communication by dorso-ventral channels situated at the lateral margins of the 
cesophagus. 
Behind the posterior extremity of the club muscle, in the region of the gills, the 
ventral of these sacs becomes broken up and finally disappears, but the dorsal sinus is 
prolonged backwards unmodified throughout the gill region, and acquires connections 
with the peribranchial sacs by vertical channels which accompany the efferent branchial 
arteries. The contents of the latter sac, therefore, may pass quite freely into the dorsal 
cesophageal sinus. 
The peribranchial or pleural sacs or sinuses} are large and well-defined bags 
covered by a fatty tissue—one to each gill) They are invariably distended with 
injection mass when the vessels have been filled from the ventricle of the heart, owing 
to the mass passing through the vascular papillze into the sacs. The gill projects quite 
freely into the sac, the walls of the two structures being here and there connected by 
fine tough threads. The posterior wall of one sac is very closely opposed to the anterior 
wall of the one behind, so that there is a double partition between any two adjacent 
gills, This partition is quite imperforate, so that no communication is here possible 
between contiguous sacs. 
There can be no doubt, as Jonannes MULzER first pointed out, that the gills are not 
morphologically within the pleural sacs, but that they are related to the sacs in 
precisely the same manner as the abdominal and thoracic viscera are to their respective 
cavities. In other words, each pleural sac consists of a visceral and a parietal layer, and 
the oill, with its afferent and efferent ducts and arteries, is really outside it. This can 
be easily demonstrated by serial sections. J. MULLER, in fact, compares the peribranchial 
sinuses of Cyclostomes not with lymph sacs properly so called, but with the pleural 
cavities of higher animals. 
The ductus cesophago-cutaneus has no sinus associated with it, but is wedged in 
between the last pleural sac of the left side and the pericardial cavity. 
The first pleural sac extends a little distance in front of the gill it encloses at the 
side of the club muscle, and is overlapped behind by the second pleural sac. It is 
only prolonged a short distance on to its efferent gill duct, the greater part of which, 
therefore, is visible without removing the sinus. 
The second sac spreads over the external surface of the root of its efferent gill duct 
for a short distance, and this tendency for the sac to extend over the efferent duct 
becomes greatly and suddenly emphasised behind the second efferent duct, so that only 
the first two ducts are visible without removing the covering sinus. The sinus system 
* Op. J. Miter, Abh, Ak. Berlin, 1834, p. 253. + J. Miinner, Abh. Ak. Berlin, 1834, p. 264 et seq. 
