ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 303 



2. Variations in the Number and Vascular Supply of the Gills. 



Amongst the many dissections of the gills which I have made, the following departures 

 from the normal number of six on each side were encountered : — 



(a) Seven gills on the left side.—The additional one was apparently the last, since it 

 was supplied by a twig from the last afferent branchial artery. Normal on the right side. 

 In this specimen the ventral aorta was continued forwards as an impaired artery on to 

 the club muscle. 



(b) Seven gills on both sides. — On the left side all the gills were of much the same 

 size, except that the middle ones were slightly the larger, and there were five afferent 

 branchial arteries. The first and last afferent branchials split quite near their origin, to 

 supply two gills each. On the right side the gills alternated with those of the left, the 

 first being situated between the first and second of the left side, and so on backwards. 

 The size of the gills was the same as on the other side, except that the last was very 

 small, and only about one-third the dimensions of the sixth. There were six afferent 

 branchials on this side, one to each gill, but the last small gill was supplied by a twig 

 from the sixth, given off near the place where it entered the sixth gill. 



(c) Seven gills on the left side. — First gill more anterior and rather more dorsal 

 than usual, and situated laterally and dorsally to the posterior end of the club muscle, 

 not reaching as far back as the latter point. Usually about half the first gill overlaps 

 the club muscle, but the forward position above described may occur without any 

 variation in the gills. Last or seventh gill much the smallest, and apparently the extra 

 gill, as it is supplied by a branch of the last or sixth afferent branchial. Ductus oesophago- 

 cutaneus present. Gills normal on the right side. 



(d) Seven gills on the left side. — Similar case in every respect to (c), except 

 that the afferent branchial for the seventh gill arose from the aorta side by side 

 with the sixth, and was not merely a branch of the latter. The extra gill is again 

 the smallest, but the disparity is not so great as in preceding case. Gills normal on 

 the right side. 



(e) Seven gills on both sides. — ^On the left the first is in a much more dorsal posi- 

 tion than the others, and lies at the side of the club-shaped muscle, which latter projected 

 slightly behind the posterior border of the gill. First afferent branchial split to supply 

 the first two gills. Apart from the usual decrease in size from before backwards, the 

 seventh gill was quite normal, and had its own afferent branchial — well separated from the 

 artery supplying the sixth gill. On the right side the first gill does not differ in position 

 from the others, but the whole seven form a graduated series of which the last is the 

 smallest, but not remarkably small. Each of the seven gills was supplied by an afferent 

 branchial artery arising separately from the aorta. 



(/) Seven gills on the left side. — First rather more dorsal than the others, and lies 

 at the side of the club muscle, which projects slightly behind it. First afferent 

 branchial splits into two to supply the first two gills. Last gill distinctly smaller than 



