90 



COLLEGE ZOOLOGY 



Appendages 



The external appendages of animals are 

 outgrowths of the body, which are used for 

 locomotion, obtaining food, protection, res- 

 piration, and many other purposes. They are 

 greatly modified for their various functions, 

 and these modifications furnish excellent 

 material for the study of homologous and 

 analogous organs. 



Homologous organs 



These are organs that are usually funda- 

 mentally similar in structure (Fig. 433) and 

 always the same in embryologic develop- 

 ment, having their origin in a common an- 

 cestral type. For example, the forelimbs of 

 the frog, the wings of birds, and the arms of 

 man serve to distinguish their bearers from 

 one another; nevertheless, these structures 

 are homologous, since they are morpholog- 

 ically equivalent. Homologous organs may 

 have similar functions, for example, the legs 

 of a man and the hindlegs of a horse; or 

 they may have different functions, for exam- 

 ple, the arms of a man and the wings of a 

 bird. 



Analogous organs 



Similar functions may make nonhomol- 

 ogous organs resemble each other. Such or- 

 gans are said to be analogous. For example, 

 the wings of butterflies and birds are analog- 

 ous because they are both used for flight, but 

 they are not homologous because they have 

 neither the same fundamental structure nor 

 the same embryonic origin. 



THE ORGANISM AS 

 A WHOLE 



We have been discussing cells, tissues, 

 organs, and systems of organs as though they 

 are independent. Nothing, however, is more 

 certain than that these parts act together as 



a unit— the organism. The cells are not in- 

 dependent. In many cases actual protoplas- 

 mic bridges connect cells as in Volvox (Fig. 

 22) and certain epithelial tissues. Even 

 closer union of cytoplasm and nuclei is ef- 

 fected in multinucleate cells, such as those 

 of skeletal muscle; this type of cell is 

 called a syncytium. Cytoplasmic connections 

 are not necessary, however, since substances 

 may pass from cell to cell by diffusion 

 through their membranes, and thus one cell 

 may have a profound influence on neighbor- 

 ing cells. Other relations between cells are 

 brought about by various organs and sys- 

 tems, such as the nerves of the nervous 

 system and the blood of the circulatory sys- 

 tem. 



Zoologists spend a large part of their time 

 studying the structures and functions of the 

 parts of animals; this is necessary for a 

 proper understanding of the whole. The 

 whole, however, differs from the sum of its 

 parts; the parts cooperate to maintain the 

 whole in its struggle to maintain itself and 

 the race. Reproduction, embryonic and 

 larval development, reactions to changes in 

 external conditions, the appearance of in- 

 herited characteristics, and organic evolu- 

 tion itself are all manifestations of the or- 

 ganism as a whole. 



THE ORIGIN OF 

 THE METAZOA 



We know very little about the relation- 

 ships of the major groups of animals, but 

 it is interesting to speculate about their 

 origin. The exact origin of the metazoans 

 is unknown, but zoologists hold the opin- 

 ion that they must have evolved from single- 

 celled organisms; since many of the cells of 

 the lower metazoans possess flagella, it seems 

 probable that the flagellates were their an- 

 cestors. Certain colonial protozoans now 

 living resemble what the metazoan ancestors 

 may have been like. Proterospongia (Fig. 

 428, p. 605) possess two very conspicuous 



