CILIARY MOTION. 147 



The cilia are microscopic, soft, transparent, colorless, homogeneous, 

 flexible organs, which, from being, in certain places, arranged in even 

 rows, have been compared to the eyelashes, and hence have received 

 their name. They are either thread-like, tapering, blunt, or flattened ; 

 they are usually attached to epithelial or epidermoid cells each cell, 

 in the most characteristic forms, bearing from six to twelve cilia. 

 They vary in length from ^^th to ^opth of an inch in man, and 

 from ysJ^th to T^oo^ n f an * ncn * n different animals, being largest 

 in the non-vertebrate marine animals, and reaching the greatest size 

 in certain Coelenterata. On the mucous membranes of the warm- 

 blooded Vertebrata they are less regular in their distribution over the 

 surface ; but on the gills of the Mollusca, on the tentacles of the 

 hydrozoa, on the lateral bands or paddles of the Beroe, and on the 

 bodies of certain infusoria, they are arranged in remarkably even 

 lines, and, in each particular case, have a very uniform length. 



The ciliary movements are, of course, only observable by aid of a 

 high magnifying power. The motion of individual cilia is difficult of 

 detection, owing to its rapidity. It is sometimes infundibuliform ; 

 that is to say, the point describes a circle in space, which forms the 

 base of a cone or funnel, the apex of which is at the attachment of the 

 moving cilium. More commonly the movement is unciform or hooked, 

 each cilium bending downwards and then straightening itself again, so 

 as to perform a lashing movement. In the case of a ciliated surface, 

 this motion appears to affect the cilia in regular succession, so that 

 the result is an undulatory movement, like that of running water, 

 moving rapidly along in constantly definite wavy lines. Nothing can 

 exceed in beauty, as a microscopic object, these waving movements of 

 the fringed and feather-like rows of the ciliated gills or branchiae of one 

 of the Mollusca (mussel, oyster). The average velocity of the ciliary 

 current in the frog's mouth is about 2 J(jth of an inch per second, the 

 average rate of the blood in the capillaries of its web being about ^th 

 of an inch per second. 



The motion of the cilia has the effect, when the animal is fixed, of 

 producing currents over the ciliated surface in the surrounding fluid 

 medium, in certain definite directions, by which not only the fluid, but 

 any small particles or microscopic living objects which it may contain, 

 are hurried past or over the surface. Indeed, the ciliary movement is 

 best observed and detected, by mixing charcoal or pigment in the fluid 

 in which these organs are examined. In the case of small aquatic 

 animals, and of ciliated ova, embryos, or gemmae of the same, the effect 

 is to cause a movement in those minute beings themselves, the cilia 

 constituting true locomotive organs. This occurs also in the Beroe, 

 the lateral ciliated bands of which are composed of flat quadrangular 

 plates, built up of large cilia, placed side by side. Most commonly, 

 the direction of the progressive movement is constant, and remains 

 unchanged even on any detached pieces of membrane ; but sometimes 

 the motion, as in the case of certain infusoria, is variable in direction, 

 and almost suggests obedience to volition. In the Beroe this is even 

 more obvious. On the gills of Mollusca the direction of the motion is 

 likewise sometimes suddenly reversed. 



