VERTEBRATE ORGANISATION 19 



Fish alone have true median fins, and usually also 

 the two pairs of fins that correspond to our arms 

 and legs. The gills, reduced in ourselves to the 

 framework of the larynx and parts of the middle 

 ear, are seen in their full development as tufts with 

 elaborate bars for their support, and with muscles for 

 conducting gulps of water in at the mouth, over the 

 tufts, and out through the gill-slits. Movements of 

 the body are carried out mainly by the muscles of 

 the back and tail, which are flattened either laterally 

 or from above downwards ; and to steady the unstable 

 fish and increase its propelling force the surface of 

 its trunk and tail is expanded into median fins. The 

 paired fins, so essential to higher animals for locomo- 

 tion, are mere steering-organs for fish, and in the 

 lowest fish of all — the lamprey and its allies — are 

 entirely dispensed with. 



Between these four divisions : mammals, reptiles 

 and birds, amphibia, and fish, there is a deep-seated 

 fundamental likeness. Differing as they do in the 

 execution of the design, these groups agree in the plan 

 of their bodily structure, and for this reason they can 

 be classed together as vertebrate animals. 



In a similar way, the bewildering variety of in- 

 vertebrate life can be reduced to groups subordinate 

 to these six larger groups : the Protozoa (simplest 

 and most primitive of animals) ; the Ccelenterates 

 (zoophytes, anemones and corals) ; the Molluscs ; the 

 Arthropods (insects, centipedes, and Crustacea) ; the 

 Echinoderms (sea-urchins, sea-lilies and starfish) ; the 



