ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 783 
the left side there are perhaps no less than three parts in the apparatus. The only 
important difference between the two sides in the reconstructed figures is that the 
connection between y* and y* is wanting on the right side; but we have seen that the 
ring condition is subject to variation. The two posterior rod-like extensions of the 
perforated plate on the left side are represented by only one on the right—but this is 
quite a minor point ; whilst we have already seen that the condition illustrated in fig. 14 
may be easily deduced from that shown in fig. 15. 
It now becomes a matter of interest to enquire how this skeleton may have been 
built up. AyeERs and Jackson suggest that on the left side it is a complex of the 
skeleton of the /ast efferent gill duct, together with that of the ductus cesophago-cutaneus. 
This may, indeed, be true for Bdellostoma ; but in Myxine it would seem as if all the gill 
ducts had a share in it, and this may explain why the apparatus is the more complex in 
Myxine. In any case, we may question whether the perforated plates of Myaine are 
represented in Bdellostoma at all, and, as Myaine is obviously the more specialised form 
as regards its gills, it would follow that this portion of the branchial skeleton is 
a neomorph of no special significance as regards the gill cartilages discovered in 
Bdellostoma by AyvERS and Jackson. It is, however, conceivable that, by a concentra- 
tion of the latter cartilages due to the confluence of the gill ducts, the branchial skeleton 
of Myxine may owe its existence. The bearing which these conclusions have on the 
branchial skeleton of cyclostomes generally is not without interest, for it would seem 
that every cyclostome must be considered on its merits; and we cannot, for example, 
say that Myxvne is intermediate between the lamprey and Bdellostoma. 
The cartilage of the branchial skeleton is histologically the feeblest of the true 
cartilages, even if it can be called such, in the whole body, being distinctly weaker than 
that of the tentacles and somewhat weaker than that of the caudal fin. The cartilage cells 
are relatively large, and are embedded in a rather delicate network which seems in places 
to be continuous and in others to consist of capsules around the cells, each of them 
independent. ‘There is only a slight deposit of cement—provided the above network 
does not, as I think, represent that substance. In this connection, compare the cartilage 
of the caudal fin. The branchial cartilage is, in fact, one of the numerous transition 
connective tissues of Myxine, and this is indicated by its staining reactions, since it 
colours neither a distinct blue nor red, but an indefinite colour suggesting both these dyes. 
It may, however, be regarded as an extreme variety of the soft cartilage. 
M. SxeLeton or THE CaupaL Fin. (Fig. 17.) 
Of the so-called “fins” of Myaine the adipose dorsal fin has no skeletal support, or, 
at the best, but a very few short detached rods extending only a very short distance 
beyond the contour of the back muscles (“‘fin” 1). This passes without a break into 
the caudal fin (‘‘fin” 2), which possesses an elaborate skeletal framework and which passes 
round the extremity of the tail and then forwards as far as the cloaca. In living 
