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 obliterating 

 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 lamellse. 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 lamellae. 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 



