THE GENERAL ANIMAL FUNCTIONS 75 



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 also. This is known 

 as sexual dimorphism (Chapter VII, 149). 



1 02. Practical Exercises. Compare the males and females of the various animal 

 types with which you are familiar. In what groups of animals does non-sexual 

 reproduction occur? Give the gist of Geddes and Thompson's theory as to the 

 origin of sexuality. What is the view based on the discoveries of Mendel? Com- 

 pare 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? 



103. Movement and the Muscular System. The desira- 

 bility 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 (20, 24) 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. 68 

 and 70). While this is found in Protozoa, it is much more 

 clearly shown in the muscular tissue (Fig. 30) 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 

 the muscle fibres are attached, and the nature of the medium 

 which must be penetrated. Many aquatic forms, though free- 

 swimming in their early stages, may become attached and give 

 up the power of locomotion in the adult condition. Such 

 attached or poorly moving forms ordinarily secrete an external 

 shell or covering into which they can withdraw for protection 

 (e.g., barnacles, many polyps). They must depend upon cur- 

 rents in the water to supply them with food . They are frequently 

 able to produce the currents by the motion of parts of the body. 

 The majority of active movers have hard parts which serve 

 as levers to which the muscles are attached. The parts of the 

 skeleton, which may be either external to the muscles or sur- 

 rounded by them, articulate with one another by a hinge or 



