THE GENERAL ANIMAL FUNCTIONS. 69 



divisions a new individual, are called conjugation or fertilisa- 

 tion, and the new individual which results is said to arise by 

 sexual reproduction. The uniting cells may be similar (as in 

 Pandorina), in which case the union is isogamous. More 

 usually the cells are different and the union is heterogarnous. 

 In the latter case the cells are called ovum and sperm (Chapter 

 IV) and are usually formed by different individuals, though 

 very often the same individual may produce both classes of cells 

 (hermaphroditism) from different regions of the germinative 

 epithelium, or in the same organ at different times. The 

 special organs in which the ova are produced are called ovaries. 

 The sperm cells are formed in testes. The individuals (that 

 is, the male and female) producing the different classes of 

 cells are often very different in other respects. This is known 

 as sexual dimorphism (Chapter VII, 145). 



99. Practical Exercises. Compare the males and females of the vari- 

 ous animal types with which you are familiar. In what groups of animals 

 does non-sexual reproduction occur? Give the gist of Geddes and Thomp- 

 son's theory as to the origin of sexuality. Compare any other theories 

 available to you. What are the conceivable advantages and disadvantages 

 of the asexual method? of the sexual? of hermaphroditism? of sexual 

 dimorphism? 



100. Movement and the Muscular System. The desir- 

 ability of motion in animals arises from the necessity of seek- 

 ing food and of escaping unfavorable influences. These con- 

 ditions constitute the most imperious stimuli to which the or- 

 ganism is subject. We have already seen ( 19, 23) that the 

 fundamental irritability and contractility of protoplasm make 

 this response possible in the simplest conditions. In somewhat 

 higher forms, specially developed protoplasmic fibrils appear, 

 such as cilia, or the fibrils in the stalk of Vorticella, in which 

 the power of contracting is strikingly manifest (see Figs. 66 

 and 68). While this is found in Protozoa, it is much mo^e 

 clearly show r n in the muscular tissue (Fig. 28) of still higher 

 animals. Locomotion varies in efficiency in different animals 

 not merely on account of varying muscular structure but in 

 accordance with the arrangement of the hard parts to which 



