STRUCTURE OF THE VERTEBRATES 5 



The student should distinguish between gills and gill slits. 

 Gills are respiratory organs which in primitive animals are con- 

 tained within the gill slits. However, other types of gills exist; 

 and in three groups of vertebrates (reptiles, birds and mammals) 

 gills are never present, even in the embryo, although slits are 

 always formed. 



Other Characteristics. In addition to the aboA^e three charac- 

 teristics, which may be called the diagnostic structures of the 

 chordates, tliere are others which are common to the vertebrates 

 and other phyla. Among the 'more important of these are the 

 following. 



1. Metamerism, or segmentation, is the division of the animal 

 into definite somites or segments, the condition arising in the 

 mesoderm as a serial repetition of parts. Among invertebrates 

 segmentation can be studied in the earthworm and the insect; 

 and can easily be seen in the lower groups of vertebrates. It is, 

 however, obscured in the more specialized forms. This apparent 

 loss is due to the development of fins or legs with their large 

 muscles which cover the fundamental structure. 



2. Coelomic Cavity, or body cavity, is found in several inver- 

 tebrate groups and in the chordates. It arises as a splitting of 

 the early mesodermal segment, the inner layer of mesoderm 

 lying next the digestive tract, the outer layer in contact with 

 the body wall. The coelom is between. 



3. Bilateral Symmetry is found in the chordates and in most 

 groups of the invertebrates. If a median plane is drawn from 

 anterior to posterior (from head to tail) either half of the animal 

 will be a mirror image of the other. Due to growth of organs 

 complete symmetry is soon lost in tlie developing embryo; but 

 in most vertebrates the skeleton, muscles and nervous system 

 keep their symmetry th.roughout life. 



4. Cephalization is the tendency of the nervous system, the 

 sensory and coordinating mechanism, to concentrate in the 

 anterior or head end. Within tlie chordate phylum there is a 

 progressive tendency toward a concentration and shortening of 

 the central nervous system. 



5. Blood Flow, or the direction of circulation, in the vertebrate 

 is the opposite of that of the invertebrate. The chordate heart is 



