76 COLLEGE ZOOLOGY 



called myonemes in the membranous coverings (p. 69). In most 

 of the higher organisms special muscle cells are differentiated for 

 performing the various movements of the body. These cells 

 possess muscle fibrils which are able to contract with great force 

 and in quick succession. The fibrils are usually of two kinds: 

 (a) cross-striated (E), and (b) smooth non-striated (F). The 

 latter form a less highly developed tissue than the former and 

 are found in the simpler inactive animals, and in those internal 

 organs of higher organisms not subject to the will of the 

 animal. 



(4) Nervous tissue (Fig. 46, G) is composed of cells which are 

 so acted upon by external physical and chemical agents that they 

 are able to perceive a stimulus, to conduct it to some other cell 

 or cells of the body, and to stimulate still other cells to activity. 

 All protoplasm is irritable; animals without nervous systems, 

 e.g. Ameba, are capable of reacting to a stimulus, but in more 

 complex organisms certain cells are specialized for the sole pur- 

 pose of performing the functions described above as character- 

 istic of nervous tissue. 



3. Organs and Systems of Organs 



An organ is an association of tissues which act together in 

 the performance of certain functions. For example, the legs of 

 human beings are organs of locomotion ; they consist of a variety 

 of tissues, including epithelial (skin), muscular (muscles), ner- 

 vous (nerves), and supporting (bones) tissues. 



The organs of different animals which occupy the same relative 

 position and have a similar origin, i.e. are morphologically equiv- 

 alent, are said to be homologous. Homologous organs may have 

 similar functions, e.g. the legs of man and the hind legs of the 

 horse, or they may have different functions, e.g. the arms of 

 man and the wings of a bird. When the organs of different ani- 

 mals perform the same functions they are said to be analogous, 

 e.g. the wing of a bird and the wing of a butterfly. In many 

 ases homologous organs are also analogous, being morphologi- 



