THE WONDERS OF LIFE 



face (ciliated epithelium). If single cells are released 

 from the group, they may live independently for some 

 time, continuing their movements and resembling free 

 infusoria. The same may be said of the travelling spores 

 of many of the algas, and of the most remarkable of all 

 ciliated cells — the spermia or spermatozoa of plants and 

 animals. 



As a rule they are cone-shaped, having an oval or 

 pear-shaped (though often also rod-shaped) head, which 

 tapers into a long and thin thread. When their lively 

 movements were first noticed in the male seminal fluid 

 (each drop of which contains millions of them) two hun- 

 dred years ago, they were thought to be real indepen- 

 dent animalcules, like the infusoria, and so obtained their 

 name of seed-animals (spermatozoa). It was a long time 

 (sixty years ago) before we learned that they are de- 

 tached glandular cells, which have the function of fer- 

 tilizing the ovum. It was discovered at the same time 

 that similar vibratory cells are found in many of the 

 plants (algce, mosses, and ferns). Many of the latter 

 (for instance, the spermatozoids of the cycadea) have, 

 instead of a few long whips, a number of short lashes 

 (cilia), and resemble the more highly developed ciliated 

 infusoria (ciliata). 



The ciliary movement of the infusoria is held to be a 

 more perfect form of vibratory movement, because the 

 many short lashes found on them are used for different 

 purposes, and have accordingly assumed different forms 

 in the division of labor. Some of the cilia are used for 

 running or swimming, others for grasping or touching, 

 and so on. In social combinations we have the ciliated 

 cells of the ciliated epithelium of the higher animals — 

 for instance, in the lungs, nostrils, and oviducts of verte- 

 brates. 



In the unicellular, non-tissue forming protists, all the 

 vital movements seem to be active functions of the plasm 



272 



