782 MR FRANK J. COLE 
in the latter the extension x* is wanting. Burne and AyrErs and Jackson also mention 
that it varies in Bdellostoma. 
On the right side (fig. 14) the branchial skeleton also varies considerably, but I 
figure and now describe its most highly differentiated condition as I have found it. The 
apparatus is at once simpler and more complex on this side—simpler in so far as the 
portion related to the ductus cesophago-cutaneus is necessarily wanting, as there is no 
ductus on this side; and more complex in as much as a complete ring is formed round ~ 
the branchial cloaca. That this ring is a secondary formation is indicated by the fact 
that in one specimen dissected it was incomplete; but there were present an anterior 
and a posterior process from the perforated plate which did not quite meet external to 
the branchial cloaca to form a perfect ring (cp. below and fig. 15). The posterior 
process of fig. 14, which passes upwards separate from and posterior to the last efferent 
gill duct, represents y* of fig. 11, whilst the bar lying external to the branchial 
cloaca is clearly y” of figs. 11 and 13. The fenestrated plate y® on the inner surface 
of the first two efferent gill ducts will naturally correspond with the same structure on 
the other side, but instead of having two large perforations it had four small ones. 
If we now turn to the reconstruction from the serial sections (fig. 15), we observe 
that the branchial skeleton here consists of two separate pieces—one external and the 
other internal to the branchial cloaca, as well as two small detached cartilages (z*, 2’). 
This condition must not be confused with the imperfect ring just described, where two 
processes from the perforated plate embraced the branchial cloaca externally. These 
two processes are doubtless similar to the two in fig. 15 seen underneath y”, and which 
seem to represent an attempt to complete the circle--not by extending round on to the 
external surface of the branchial cloaca, as above described, but by fusing with y*. It 
therefore seems as if the circle may be formed in more ways than one. In fig. 15, y” 
and y* are related rather to the branchial cloaca than to individual gill ducts,* except 
that the last efferent gill duct passes between y* and the posterior extension of y’. 
The two latter processes serve to support the ventrally directed portion of the branchial 
cloaca just above its external opening. The separate cartilage z* curls round under the 
ventral edge of the branchial cloaca as if to fuse with y”, but it does not do so. It is 
evidently a detached portion of the backward blunt process given off from the perforated 
plate in front. The posterior downward process from the same plate is separated from 
y* by the ventral extension of the branchial cloaca. The detached cartilage 2” is 
situated behind and above the external branchial opening, and, perhaps, represents the 
extremity of the line of chondral deposit forming y’. 
A comparison of figs. 13 and 15 shows at once that the corresponding parts of the — 
two sides are essentially similar, and further, that the efferent gill duct portion of the 
branchial skeleton may itself be a complex formed of at least two pieces; and hence on 
* It is, perhaps, rather a refinement of description to associate any part of the branchial skeleton with individual 
gill ducts, since, apart from the ductus cesophago-cutaneus portion, its function on both sides and in all cases is to 
strengthen the wall of the branchial cloaca, and it is possible that all the gill ducts have contributed to it. 
 -a 
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