90 
These structures, the convex surface of the gill arch, the 
proximal ends of the gill rays and the ligaments, form a 
tunnel through which pass the efferent branchial vessels 
and certain of the nerves of the gill. A very definite 
little muscle takes origin in the cartilaginous connecting 
portion of every two gill rays, passes obliquely downwards 
into the tissues of the opposite gill filament, and is in- 
serted into the upper portion of its gill ray. This is part 
of an extremely pretty mechanical arrangement. It has 
been stated that each half filament is in section an isosceles 
triangle, and that the two series dove-tail into each other 
on account of their alternate arrangement. Obviously the 
contraction of the little muscles described above must have 
the effect of approximating the two series and ebliterating 
the spaces between all the separate filaments; conversely 
the relaxation of the muscles and the elastic recoil of the 
gill rays must separate all the filaments attached to a 
single arch, leaving a space between each two, and this is 
effected without any alteration in the total length of the 
gill. It seems extremely probable, though we have no 
experimental evidence on the point, that these movements 
do actually take place in life, and that they aid in the 
movement through the gill of the respiratory water. 
Text-fig. 4, C. is a longitudinal section through a gill 
filament in a plane at right-angles with that of 4, A. It 
shews that each filament consists of a flattened axis which 
bears on either side a close-set series of lamelle. The 
axis is strengthened by dense connective tissue, and con- 
tains the gill ray and the filamentar blood vessels. Text- 
fig. 4, B. is a transverse section of a double filament 
between the centres of two lamelle. It shews the position 
of the axis and the gill ray. 
If the branchial blood vessels are injected from the 
ventral aorta, a series of vessels become apparent on the 
