MORPHOLOGY OF ANIMALS 



sexes differ not only with respect to the sexual apparatus 

 but also in many other regards, which are known as secon- 

 dary sexual characters; when such differences are very 

 marked they constitute what is known as sexual dimor- 

 phism. In such cases the male is sometimes very degen- 

 erate in form, being occasionally only a small fraction of 

 the size of the female and entirely lacking alimentary 

 canal, sense organs, and nervous system (rudimentary 

 males of rotifers, barnacles, etc.). 



Asexual reproduction^ or monogony^ consists in the for- 

 mation of new individuals by division of an old one. In 

 one-celled organisms and in the constituent cells of higher 

 animals this takes the form of cell division or the partial 

 division of cell aggregates. In the lower Metazoa asexual 

 reproduction is not limited to cell division, but the entire 

 body or portions of it may undergo constriction and sub- 

 sequent partial or complete division, thus giving rise to 

 new individuals. This division may be into equal parts, in 

 which case it is called fission ; or into unequal parts, when 

 it is known as budding or gemmation. In animals which 

 reproduce both sexually and asexually there is more or 

 less regular alternation of one method with the other; this 

 is known as alternation of generations or metagenesis. The 

 alternation of amphigony with parthenogenesis is called 

 heterogony. 



9. Nervous System 



Sensation and coordination are manifestations of pro- 

 toplasmic irritability, or that capacity of receiving and 

 responding to stimuli characteristic of every cell. Animals, 

 even the simplest, are sensitive to a variety of stimuli, 



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