82 COMPARATIVE ANATOMY 



wards, and its edge is formed by a border of highly refracting 

 contractile substance differing in appearance from the cytoplasm 

 of which the rest of the cell -body is composed. From this 

 border the cilia project into the cavity of the pharynx. The 

 cell tapers gradually inwards, its inner end being fine and 

 pointed or drawn out into two or more irregular processes 

 which rest upon a basement membrane. The nucleus is 

 relatively large, placed in about the middle of the length of 

 the cell, which is rather bulged out by it. The ciliated cells 

 found on the gills of oysters and mussels are larger and have 

 longer cilia than those of the frog's palate, and afford a 

 very convenient means of studying ciliary action. The great, 

 flat, plate-like gills of these animals are covered with a ciliated 

 epithelium, whose cilia, constantly in motion, drive currents 

 of water through the gills in a definite direction. The cells 

 are long and columnar, tapering internally, and with large 

 nuclei. The refracting border on the free surfaces of the cells 

 may be seen to be made up of a number of minute refractive 

 knobs placed close together. Each knob bears a cilium on 

 its outer side, and internally it is prolonged into a fine varicose 

 fibril which runs down into the cell-body past the nucleus, 

 eventually uniting with similar fibrils from the other knobs 

 to form a single thread which runs into the pointed inner 

 extremity of the cell (fig. 14, D 3). 



Several phenomena may be noticed in a fragment of the gill 

 of an oyster or mussel examined under the microscope. The 

 ciliary motion appears to sweep over the surface in waves, just 

 as a field of corn is thrown into waves by the wind passing 

 over it, arid the waves always pass in the same direction. The 

 ciliary motion continues for some considerable time in the 

 excised piece of gill, and even in small fragments or single 

 cells which have been broken away with needles. This shows 

 that the activity of the cilia is not due to impulses reaching 

 the cells by nerves from the nervous centres, but that it is 

 dependent on the activity of the cells themselves, which 

 remain living for some time after their removal from the body. 

 If the temperature is lowered the activity of the cilia is 

 diminished, and eventually is stopped altogether at some 

 degrees below freezing point, but it is revived on warming. 

 Up to a certain point warmth increases the activity of the 

 cilia, but above a certain point it retards, and soon stops their 



