98 CILIARY MOTIONS. 



portion of the gills of the fresh-water mussel (Anodon cygneus), 

 it will be found to exhibit cilia and their motions to great 

 advantage ; viewed with a quarter of an inch object-glass, the 

 Fig. 67.* cilia are then seen to consist of 



delicate filaments like hairs, set more 

 or less regularly in rows, and moved 

 with rapidity. In this mollusk, the 

 cilia are about 1-1 00th of a line in 

 length, as seen in (c, c), and are set 

 upon rounded cells (b, b), as upon 

 bulbs ; their motion is hook-like, or, 

 in other words, the point of each cilia successively bends to- 

 wards its base, and is rapidly stretched out again. These 

 motions are performed more or less vividly in different ani- 

 mals, and in different states of the same animal. The infusoria 

 (fig. 171) exhibit this phenomenon in an admirable manner; 

 the surface of their bodies is covered with rows of cilia, which 

 perform various motions ; a great number of the embryos of 

 sponges, polyps, acalephae (fig. 368), and mollusca are covered 

 with vibritile cilia during the first periods of their existence, and 

 these microscopic filaments play an important part in many of the 

 organs of the invertebrata. The sides of the bodies of beroes, and 

 the tentacula of medusae, exhibit these motions ; they are seen 

 in the interior of the tentacula of Actinia and other ZOAN- 

 THID^E ; on the oral lobes of the rotifera (fig. 1 72) ; on the 

 exterior of the tentacula of Flustra, Alcyonella, and other 

 BETOZOID^! (fig. 1 75) ; the membrane lining the test of 

 urchins, CIDABID^:, and sea-stars, ASTEBIADJB ; the anterior 

 parts of the bodies of the fresh-water mollusca, and the 

 branchiae of all univalve and bivalved genera, with those of 

 cirrhipedes and Crustacea (fig. 370), are provided with vibra- 

 tile cilia. 



In the vertebrated animals ciliary motions are seen on many 

 parts of their bodies. On the mucous membrane covering 

 the gills of the tadpoles of frogs and salamanders, and on the 

 respirating organs as well as on the membrane lining the 

 mouth, fauces, and nasal passages of amphibia, reptiles, 

 birds, and mammals. Ciliary motions are intended to renew 

 the stratum of water or air bathing the surface covered by these 



* There ought to be no space betwixt the epithelial cylinders that sup- 

 port the cilia, and the cilia themselves, as in the above figure, which is a 

 mistake of the artist ; they are immediately sessile upon the epithelium, as 

 in the plan (fig. 68), 



