STRUCTURE OF ITS GILLS. 53 



the microscope. Witli a power of 220 diameters, I 

 distinctly perceived a large lens, a glassy coat invest- 

 ing this, which itself was huried for more than half 

 its volume in an investiture apparently granular of 

 a yellowish brown colour, having an ill-defined circle 

 near its anterior side, of a blackish hue. Under 

 pressure with the compressorium, the lens was mani- 

 festlv circular ; the coloured socket discharo-ed dark 

 granules, and from the darkest part a deep crimson 

 pigment, which did not appear to be granular 

 (See Plate III. fig. 5.) 



I submitted portions of the gills also to the same 

 magnifying power. Each of the four lamina consists 

 of a vast number of straight slender transparent fila- 

 ments, evidentlv tubular, and about r^th of an inch 

 in diameter, arranged side by side ; or rather of o?ie 

 filament, excessively long, reverted upon itself 

 again and again, at both the free and the at- 

 tached end of the laminse, throughout its whole ex- 

 tent. This repeated filament is armed on each of two 

 opposite sides mth a line of vibrating cilia, the two 

 lines moving in contrary directions ; by the action of 

 which a current of water is made continually to flow 

 up and do^^Tl each of these delicate filaments ; so that 

 the blood which circulates in their interior (for they 

 are doubtless blood-vessels) is continually exposed 

 throughout this its long and tortuous course to the 

 action of oxygen. 



Like all organic .functions, the action of these cilia 

 is not under the will of the animal. It is said that if 

 during life a small portion of the gills be cut ofl*, the 

 motion of the ciha will convey the fragment swiftly away, 



