THE NATURE OF CILIA 35 



scopically small creatures, which even before birth 

 spin round inside their egg-capsules by the aid of 

 these cilia, and when out in the water glide through 

 it by their aid ; and the bodies of zoophytes and corals, 

 sea-mats and sea-mosses, of oysters, scallops, and 

 cockles, which appear so hard and motionless when old, 

 are rippled when young by the constant play of the 

 waving bands of cilia that, aided by the currents, 

 enable them to swim from the depths to the surface 

 and from the open sea to the shore. 



Cilia such as these are not confined to the surface 

 of animals, nor is their only use that of locomotion. 

 If we buy a pennyworth of mussels from a fishmonger, 

 and place them in a dish of sea-water, the shellfish 

 will soon open their valves and protrude their yellow 

 skin fringed with tentacles. If a little Indian ink is 

 added by a pipette, a movement in the water previously 

 invisible will be rendered clear. The ink will be 

 drawn between the tentacles into the valves ; some- 

 thing is at work inhaling a current of water. If now 

 we open one of the mussels and cut off a piece of one 

 of the yellow gills and place it in the water, the reason 

 of the current will be made clear. The morsel of 

 gill begins to move steadily along the bottom of the 

 dish, as though endowed with independent life, and, 

 if held up to the light in a glass dish and examined 

 with a magnifying-glass, the filmy margin due to 

 innumerable cilia can be seen. These vibrating rods, 

 which when held by the weight of the intact gill 

 sucked in a current of water, now, when attached to 



