ELEMENTARY VITAL PHENOMENA 249 



precedes that of each lower one (Fig. 108). In other words, the 

 uppermost cilium gives the sign to the others ; if the uppermost 

 one is at rest, the others rest ; if it contracts, they also contract in 

 order ; and this is true not only of the cilia of the single cell, but, 



FIG. 108. Motion of a row of cilia, in profile. 



in ciliated epithelium, of the cilia of all the cells in a row. In this 

 manner there occurs an extremely delicate and regular play of the 

 cilia, which has fascinated many observers and gives the impression 

 of regular waves passing over the ciliated row, somewhat as the 

 wind sweeps over a field of grain. When several parallel rows of 

 cilia are present, the cilia standing beside one another in adjacent 

 rows beat synchronously, just as the fibrillse lying beside one 

 another in a muscle-fibre contract at the same time. 



The phases of movement of the individual cilium can best be 

 studied in the swimming-plates of the Ctenophora. 1 The body of 

 these remarkable animals consists of a delicate transparent jelly, 

 and possesses eight stripes or ribs (Fig. 109) extending from one 



FIG. 109. Beroe ovata, a ctenophore, natural size. Of the eight ribs or rows of swimming-plates 

 extending from the upper (sense-)pole to the lower (mouth-) pole, only the four rows of one 

 side are to be seen, two from the front, and two from the side. 



pole to the other ; each rib consists of a row of plates, the swim- 

 ming-plates, that lie upon one another like tiles upon a roof. 

 Each swimming-plate is about 2 mm. long, and consists of a con- 

 siderable number of cilia, cemented together, which belong to the 

 cell-bodies lying beneath. On account of their extraordinary 



1 Cf. Verworn ('90, 2). 



