PHYSIOLOGICAL 235 



active in the interior of excretory organs, and they line the interior 

 of the genital ducts of most female animals, thus aiding in the 

 descent of the ova and e\en in the ascent of the sperms. 



In animals with much chitin there seems not unnaturally some- 

 thing physiologically antithetic to the development of cilia. Thus 

 they are absent throughout the Arthropod phylum, with perhaps 

 the single exception of Peripatus, which has cilia in its kidney- 

 tubes or nephridia. They are also absent in the Round Worms or 

 Nematodes, which have a chitinoid cuticle, with perhaps the single 

 exception of certain vibratile tags found in the lining of the female 

 genital ducts. 



What has been said of cilia applies also to flagella, which have 

 an undulating movement, whereas cilia are alternately bent and 

 straightened again. In sleeping-sickness organisms, which belong to 

 the order of Flagellate Infusorians, a flagellum is expanded into an 



Fig. 44. 

 A Single Smooth or Unstriped Muscle-cell, Slowly Contracting. N, the nucleus . 



undulating membrane which extends along the whole length of the 

 cell, and includes eight fine contractile threads or myonemes. At one 

 end the flagellum projects freely. In this and in many other cases 

 there appears to be some diffuse contractility throughout the 

 whole unit. 



Muscle. — ^There are two chief kinds of muscular tissue among 

 animals. There is unstriped, smooth, plain, or involuntary muscle, 

 which consists of homogeneous spindle-shaped cells closely fitting 

 accordingly between one another, each with a single nucleus. These 

 unstriped muscle-cells are found in sluggish animals, such as tape- 

 worms and sea-squirts, and in the slowly contracting parts of 

 animals, such as the walls of the food- canal, the walls of the bladder, 

 the walls of the arteries, and the skin around the roots of the hairs. 

 The zoologically important fact is that smooth muscle is slowly 

 contracting, and thus more primitive. Its contractions are usually 

 outside the control of the will, hence the term "involuntary", but 



