STRUCTURE OF ITS GILLS. 53 



the iiiicioscope. With a power of 220 diameters, I 

 distinct] y 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 hrown coloui', having an ill-defined circle 

 near its anterior side, of a blackish hue. Under 

 pressure with the compressoiium, the lens was mani- 

 festly circular ; the coloured socket discharged dark 

 granules, hiuI from the darkest part a deep crimson 

 pigment, which did not appear to he granular 

 (S'ee Plate III. fig. 5.) 



I submitted portions of the gills also to the same 

 magnifying power. Eacli of the four laminee consists 

 of a vast number of straight slender transparent fila- 

 ments, evidently tubular, and about y^th of an inch 

 in diameter, arranged side by side ; or rather of one 

 jilament, excessively long, reverted upon itself 

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

 tached end of the laminae, throughout its whole ex- 

 tent. This repeated filament is armed on each of two 

 opposite sides with a line of vibrating cilia, the two 

 lines mo^•ing in contrary directions ; by the action of 

 which a cuiTfiit of water is made continually to flow 

 up and down each of these delicate filaments ; so that 

 the ])lood which circulates in their interior (for they 

 are doubtless blood-vessels) is continually exposed 

 throughout this its long and tortuous course to the 

 acti(m of oxygen. 



Like all organic functions, the a(;tion 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 off, the 

 motion of the cilia will convey the fragment swiftly away. 



